diff --git a/langserver/internal/lsp/service.go b/langserver/internal/lsp/service.go
index 593228f..2d3bb1d 100644
--- a/langserver/internal/lsp/service.go
+++ b/langserver/internal/lsp/service.go
@@ -435,11 +435,23 @@ var completionItemKindName = map[CompletionItemKind]string{
 	CIKTypeParameter: "typeParameter",
 }
 
+type MarkupKind string
+
+const (
+	MKPlainText MarkupKind = "plaintext"
+	MKMarkdown  MarkupKind = "markdown"
+)
+
+type MarkupContent struct {
+	Kind  MarkupKind `json:"kind"`
+	Value string     `json:"value"`
+}
+
 type CompletionItem struct {
 	Label               string             `json:"label"`
 	Kind                CompletionItemKind `json:"kind,omitempty"`
 	Detail              string             `json:"detail,omitempty"`
-	Documentation       string             `json:"documentation,omitempty"`
+	Documentation       MarkupContent      `json:"documentation,omitempty"`
 	SortText            string             `json:"sortText,omitempty"`
 	FilterText          string             `json:"filterText,omitempty"`
 	InsertText          string             `json:"insertText,omitempty"`
diff --git a/langserver/internal/source/completion/builtin_function.go b/langserver/internal/source/completion/builtin_function.go
index 03321bb..4a79f6c 100644
--- a/langserver/internal/source/completion/builtin_function.go
+++ b/langserver/internal/source/completion/builtin_function.go
@@ -25,10 +25,13 @@ func (c *completor) completeBuiltinFunction(ctx context.Context, parsedFile file
 	for _, f := range function.BuiltInFunctions {
 		if strings.HasPrefix(f.Name, incompleteColumnName) {
 			result = append(result, CompletionItem{
-				Kind:          lsp.CIKFunction,
-				NewText:       f.Name,
-				TypedPrefix:   incompleteColumnName,
-				Documentation: f.Description,
+				Kind:        lsp.CIKFunction,
+				NewText:     f.Name,
+				TypedPrefix: incompleteColumnName,
+				Documentation: lsp.MarkupContent{
+					Kind:  lsp.MKMarkdown,
+					Value: f.Description,
+				},
 			})
 		}
 	}
diff --git a/langserver/internal/source/completion/completion.go b/langserver/internal/source/completion/completion.go
index 3fa4b69..085b393 100644
--- a/langserver/internal/source/completion/completion.go
+++ b/langserver/internal/source/completion/completion.go
@@ -173,10 +173,13 @@ func (c *completor) completeTableScanField(ctx context.Context, tableScanNode *r
 	if strings.HasPrefix(tableScanNode.Alias(), incompleteColumnName) {
 		return []CompletionItem{
 			{
-				Kind:          lsp.CIKField,
-				NewText:       tableScanNode.Alias(),
-				Documentation: tableScanNode.Table().FullName(),
-				TypedPrefix:   incompleteColumnName,
+				Kind:    lsp.CIKField,
+				NewText: tableScanNode.Alias(),
+				Documentation: lsp.MarkupContent{
+					Kind:  lsp.MKPlainText,
+					Value: tableScanNode.Table().FullName(),
+				},
+				TypedPrefix: incompleteColumnName,
 			},
 		}
 	}
@@ -277,10 +280,13 @@ func (c *completor) completeProjectForTablePath(ctx context.Context, param table
 		}
 
 		result = append(result, CompletionItem{
-			Kind:          lsp.CIKModule,
-			NewText:       p.ProjectId,
-			Documentation: p.Name,
-			TypedPrefix:   param.ProjectID,
+			Kind:    lsp.CIKModule,
+			NewText: p.ProjectId,
+			Documentation: lsp.MarkupContent{
+				Kind:  lsp.MKPlainText,
+				Value: p.Name,
+			},
+			TypedPrefix: param.ProjectID,
 		})
 	}
 
@@ -300,10 +306,13 @@ func (c *completor) completeDatasetForTablePath(ctx context.Context, param table
 		}
 
 		result = append(result, CompletionItem{
-			Kind:          lsp.CIKModule,
-			NewText:       d.DatasetID,
-			Documentation: fmt.Sprintf("%s.%s", d.ProjectID, d.DatasetID),
-			TypedPrefix:   param.DatasetID,
+			Kind:    lsp.CIKModule,
+			NewText: d.DatasetID,
+			Documentation: lsp.MarkupContent{
+				Kind:  lsp.MKPlainText,
+				Value: fmt.Sprintf("%s.%s", d.ProjectID, d.DatasetID),
+			},
+			TypedPrefix: param.DatasetID,
 		})
 	}
 
@@ -323,10 +332,13 @@ func (c *completor) completeTableForTablePath(ctx context.Context, param tablePa
 		}
 
 		result = append(result, CompletionItem{
-			Kind:          lsp.CIKModule,
-			NewText:       t.TableID,
-			Documentation: fmt.Sprintf("%s.%s.%s", t.ProjectID, t.DatasetID, t.TableID),
-			TypedPrefix:   param.TableID,
+			Kind:    lsp.CIKModule,
+			NewText: t.TableID,
+			Documentation: lsp.MarkupContent{
+				Kind:  lsp.MKPlainText,
+				Value: fmt.Sprintf("%s.%s.%s", t.ProjectID, t.DatasetID, t.TableID),
+			},
+			TypedPrefix: param.TableID,
 		})
 	}
 
@@ -426,10 +438,13 @@ type columnInterface interface {
 
 func createCompletionItemFromColumn(column columnInterface, incompleteColumnName string) CompletionItem {
 	return CompletionItem{
-		Kind:          lsp.CIKField,
-		NewText:       column.Name(),
-		Documentation: column.Type().TypeName(types.ProductExternal),
-		TypedPrefix:   incompleteColumnName,
+		Kind:    lsp.CIKField,
+		NewText: column.Name(),
+		Documentation: lsp.MarkupContent{
+			Kind:  lsp.MKPlainText,
+			Value: column.Type().TypeName(types.ProductExternal),
+		},
+		TypedPrefix: incompleteColumnName,
 	}
 }
 
@@ -439,9 +454,12 @@ func createCompletionItemFromSchema(schema *bq.FieldSchema, incompleteColumnName
 		detail += "\n" + schema.Description
 	}
 	return CompletionItem{
-		Kind:          lsp.CIKField,
-		NewText:       schema.Name,
-		Documentation: detail,
-		TypedPrefix:   incompleteColumnName,
+		Kind:    lsp.CIKField,
+		NewText: schema.Name,
+		Documentation: lsp.MarkupContent{
+			Kind:  lsp.MKPlainText,
+			Value: detail,
+		},
+		TypedPrefix: incompleteColumnName,
 	}
 }
diff --git a/langserver/internal/source/completion/completion_test.go b/langserver/internal/source/completion/completion_test.go
index b3b976a..1dd7da3 100644
--- a/langserver/internal/source/completion/completion_test.go
+++ b/langserver/internal/source/completion/completion_test.go
@@ -48,14 +48,20 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "name",
-					Documentation: "STRING",
+					Kind:    lsp.CIKField,
+					NewText: "name",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "STRING",
+					},
 				},
 			},
 		},
@@ -76,9 +82,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -108,9 +117,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "name",
-					Documentation: "STRING",
+					Kind:    lsp.CIKField,
+					NewText: "name",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "STRING",
+					},
 				},
 			},
 		},
@@ -132,9 +144,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INT64",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INT64",
+					},
 				},
 				{ // TODO: Refactoring test
 					Kind:    lsp.CIKField,
@@ -159,10 +174,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -183,10 +201,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -213,9 +234,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -248,9 +272,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -277,10 +304,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -307,9 +337,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -336,9 +369,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INT64",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INT64",
+					},
 				},
 			},
 		},
@@ -359,9 +395,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -382,9 +421,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -405,10 +447,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -429,9 +474,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INT64",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INT64",
+					},
 				},
 			},
 		},
@@ -451,10 +499,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "table",
-					Documentation: "project.dataset.table",
-					TypedPrefix:   "t",
+					Kind:    lsp.CIKField,
+					NewText: "table",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.table",
+					},
+					TypedPrefix: "t",
 				},
 			},
 		},
@@ -497,9 +548,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -520,9 +574,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -543,9 +600,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -566,9 +626,12 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
 				},
 			},
 		},
@@ -589,10 +652,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -613,10 +679,13 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "i",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "i",
 				},
 			},
 		},
@@ -646,28 +715,40 @@ func TestProject_CompleteFromSQLContext(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description",
-					TypedPrefix:   "",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description",
+					},
+					TypedPrefix: "",
 				},
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "id",
-					Documentation: "INTEGER\nid description2",
-					TypedPrefix:   "",
+					Kind:    lsp.CIKField,
+					NewText: "id",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "INTEGER\nid description2",
+					},
+					TypedPrefix: "",
 				},
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "t1",
-					Documentation: "project.dataset.table",
-					TypedPrefix:   "",
+					Kind:    lsp.CIKField,
+					NewText: "t1",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.table",
+					},
+					TypedPrefix: "",
 				},
 				{
-					Kind:          lsp.CIKField,
-					NewText:       "t2",
-					Documentation: "project.dataset.table2",
-					TypedPrefix:   "",
+					Kind:    lsp.CIKField,
+					NewText: "t2",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.table2",
+					},
+					TypedPrefix: "",
 				},
 			},
 		},
@@ -743,14 +824,20 @@ func TestProject_CompleteFromSQLContext_FromClause(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "1table",
-					Documentation: "project.dataset.1table",
+					Kind:    lsp.CIKModule,
+					NewText: "1table",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.1table",
+					},
 				},
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "2table",
-					Documentation: "project.dataset.2table",
+					Kind:    lsp.CIKModule,
+					NewText: "2table",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.2table",
+					},
 				},
 			},
 		},
@@ -775,9 +862,12 @@ func TestProject_CompleteFromSQLContext_FromClause(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "table20230622",
-					Documentation: "project.dataset.table20230622",
+					Kind:    lsp.CIKModule,
+					NewText: "table20230622",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset.table20230622",
+					},
 				},
 			},
 		},
@@ -805,14 +895,20 @@ func TestProject_CompleteFromSQLContext_FromClause(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "dataset1",
-					Documentation: "project.dataset1",
+					Kind:    lsp.CIKModule,
+					NewText: "dataset1",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset1",
+					},
 				},
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "dataset2",
-					Documentation: "project.dataset2",
+					Kind:    lsp.CIKModule,
+					NewText: "dataset2",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project.dataset2",
+					},
 				},
 			},
 		},
@@ -839,16 +935,22 @@ func TestProject_CompleteFromSQLContext_FromClause(t *testing.T) {
 			},
 			expectCompletionItems: []CompletionItem{
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "project1",
-					Documentation: "project name",
-					TypedPrefix:   "p",
+					Kind:    lsp.CIKModule,
+					NewText: "project1",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project name",
+					},
+					TypedPrefix: "p",
 				},
 				{
-					Kind:          lsp.CIKModule,
-					NewText:       "project2",
-					Documentation: "project name",
-					TypedPrefix:   "p",
+					Kind:    lsp.CIKModule,
+					NewText: "project2",
+					Documentation: lsp.MarkupContent{
+						Kind:  lsp.MKPlainText,
+						Value: "project name",
+					},
+					TypedPrefix: "p",
 				},
 			},
 		},
diff --git a/langserver/internal/source/completion/item.go b/langserver/internal/source/completion/item.go
index 5177f1e..610b185 100644
--- a/langserver/internal/source/completion/item.go
+++ b/langserver/internal/source/completion/item.go
@@ -5,7 +5,7 @@ import "github.com/kitagry/bqls/langserver/internal/lsp"
 type CompletionItem struct {
 	Kind          lsp.CompletionItemKind
 	NewText       string
-	Documentation string
+	Documentation lsp.MarkupContent
 	TypedPrefix   string
 }