diff --git a/__test__/resource.test.ts b/__test__/resource.test.ts
index ec5bff0b..5fd970ce 100644
--- a/__test__/resource.test.ts
+++ b/__test__/resource.test.ts
@@ -1,6 +1,14 @@
 import * as res from "../src/options/logic/resource";
 
 test("", () => {
+  expect(res.decideInitialLanguage([])).toEqual("en");
+  expect(res.decideInitialLanguage(["en", "ja"])).toEqual("en");
+  expect(res.decideInitialLanguage(["ja", "en"])).toEqual("ja");
+  expect(res.decideInitialLanguage(["fr", "ja", "en"])).toEqual("ja");
+  expect(res.decideInitialLanguage(["en-US", "ja"])).toEqual("en");
+  expect(res.decideInitialLanguage(["en-UK", "ja"])).toEqual("en");
+  expect(res.decideInitialLanguage(["ja-JP", "en"])).toEqual("ja");
+
   res.setLang("ja");
   expect(res.get("selectDictFile")).toEqual("辞書ファイルを選択してください。");
   expect(res.get("finishRegister", { count: 999 })).toEqual("登録完了(999語)");