diff --git a/webapp/api/index.ts b/webapp/api/index.ts
index 2ccae13..b881b3a 100644
--- a/webapp/api/index.ts
+++ b/webapp/api/index.ts
@@ -8,6 +8,7 @@ import { IdpRepository } from '../repository/idp'
 
 import cbRoute from './cb'
 import goRoute from './go'
+import loginRoute from './login'
 import oauthRoute from './oauth'
 import tokenRoute from './token'
 
@@ -32,6 +33,7 @@ app.use(async (c, next) => {
 app.route('/token', tokenRoute)
 app.route('/go', goRoute)
 app.route('/cb', cbRoute)
+app.route('/login', loginRoute)
 app.route('/oauth', oauthRoute)
 
 export default app
diff --git a/webapp/api/login/github.ts b/webapp/api/login/github.ts
new file mode 100644
index 0000000..3a63023
--- /dev/null
+++ b/webapp/api/login/github.ts
@@ -0,0 +1,156 @@
+import { zValidator } from '@hono/zod-validator'
+import { createAppAuth } from '@octokit/auth-app'
+import { Hono } from 'hono'
+import { HonoEnv } from 'load-context'
+import { Octokit } from 'octokit'
+import { binaryToBase64 } from 'utils/convert-bin-base64'
+import cookieSessionStorage from 'utils/session.server'
+import { z } from 'zod'
+
+const app = new Hono<HonoEnv>()
+
+app.get('/', async c => {
+  const state = binaryToBase64(crypto.getRandomValues(new Uint8Array(30)))
+
+  const { getSession, commitSession } = cookieSessionStorage(c.env)
+  const session = await getSession(c.req.raw.headers.get('Cookie'))
+  session.flash('state', state)
+  c.header('Set-Cookie', await commitSession(session))
+
+  // ref: https://docs.github.com/ja/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps
+  const oauthUrl = new URL('https://github.com/login/oauth/authorize')
+  const oauthParams = new URLSearchParams()
+  oauthParams.set('client_id', c.env.GITHUB_OAUTH_ID)
+  oauthParams.set('redirect_uri', `${c.env.CF_PAGES_URL}/login/github/callback`)
+  oauthParams.set('scope', 'read:user')
+  oauthParams.set('state', state)
+  oauthParams.set('allow_signup', 'false')
+
+  return c.redirect(oauthUrl.toString() + '?' + oauthParams.toString(), 302)
+})
+
+app.all('/', async c => {
+  return c.text('method not allowed', 405)
+})
+
+interface GitHubOAuthTokenResponse {
+  access_token: string
+  scope: string
+  token_type: string
+}
+
+// TODO: cookieSessionStorage の userId 以外は使ってないので消す
+app.get(
+  '/callback',
+  zValidator('query', z.object({ code: z.string(), state: z.string() })),
+  async c => {
+    const { code, state } = c.req.valid('query')
+
+    const { getSession, commitSession } = cookieSessionStorage(c.env)
+    const session = await getSession(c.req.raw.headers.get('Cookie'))
+
+    if (state !== session.get('state')) {
+      c.header('Set-Cookie', await commitSession(session))
+      return c.text('state mismatch', 400)
+    }
+
+    const continueTo = session.get('continue_to')
+    if (!continueTo) {
+      c.header('Set-Cookie', await commitSession(session))
+      return c.text('continue_to not found', 400)
+    }
+
+    const { access_token } = await fetch(
+      'https://github.com/login/oauth/access_token',
+      {
+        method: 'POST',
+        headers: {
+          'Content-Type': 'application/json',
+          Accept: 'application/json',
+        },
+        body: JSON.stringify({
+          client_id: c.env.GITHUB_OAUTH_ID,
+          client_secret: c.env.GITHUB_OAUTH_SECRET,
+          code,
+        }),
+      },
+    )
+      .then(res => res.json<GitHubOAuthTokenResponse>())
+      .catch(() => ({ access_token: null }))
+
+    if (!access_token) {
+      return c.text('invalid code', 400)
+    }
+
+    // ----- メンバーの所属判定 ----- //
+    const userOctokit = new Octokit({ auth: access_token })
+    const appOctokit = new Octokit({
+      authStrategy: createAppAuth,
+      auth: {
+        appId: c.env.GITHUB_APP_ID,
+        privateKey: atob(c.env.GITHUB_APP_PRIVKEY),
+        installationId: '41674415',
+      },
+    })
+
+    const { data: user } = await userOctokit.request('GET /user')
+
+    let isMember = false
+    try {
+      const checkIsOrgMemberRes = await appOctokit.request(
+        'GET /orgs/{org}/members/{username}',
+        {
+          org: 'saitamau-maximum',
+          username: user.login,
+        },
+      )
+      isMember = (checkIsOrgMemberRes.status as number) === 204
+    } catch {
+      isMember = false
+    }
+
+    if (!isMember) {
+      // いったん member じゃない場合はログインさせないようにする
+      // TODO: IdP が出来たらこっちも対応できるようにする
+      c.header('Set-Cookie', await commitSession(session))
+      return c.text('not a member', 403)
+    }
+
+    // すでになければ DB にユーザー情報を格納
+    const oauthConnInfo = await c.var.idpClient.getUserIdByOauthId(
+      1,
+      String(user.id),
+    )
+    if (!oauthConnInfo) {
+      const uuid = crypto.randomUUID().replaceAll('-', '')
+      // とりあえず仮情報で埋める
+      await c.var.idpClient.createUserWithOauth(
+        {
+          id: uuid,
+          display_name: user.login,
+          profile_image_url: user.avatar_url,
+        },
+        {
+          user_id: uuid,
+          provider_id: 1,
+          provider_user_id: String(user.id),
+          email: user.email,
+          name: user.login,
+          profile_image_url: user.avatar_url,
+        },
+      )
+      session.set('user_id', uuid)
+    } else {
+      session.set('user_id', oauthConnInfo.user_id)
+    }
+
+    c.header('Set-Cookie', await commitSession(session))
+    return c.redirect(continueTo, 302)
+  },
+)
+
+app.all('/callback', async c => {
+  return c.text('method not allowed', 405)
+})
+
+export default app
diff --git a/webapp/api/login/index.ts b/webapp/api/login/index.ts
new file mode 100644
index 0000000..efd011b
--- /dev/null
+++ b/webapp/api/login/index.ts
@@ -0,0 +1,13 @@
+import { Hono } from 'hono'
+
+import { HonoEnv } from '../../load-context'
+
+import loginGithubRoute from './github'
+import loginIndexRoute from './login'
+
+const app = new Hono<HonoEnv>()
+
+app.route('/', loginIndexRoute)
+app.route('/github', loginGithubRoute)
+
+export default app
diff --git a/webapp/api/login/login.ts b/webapp/api/login/login.ts
new file mode 100644
index 0000000..ce665c6
--- /dev/null
+++ b/webapp/api/login/login.ts
@@ -0,0 +1,48 @@
+import { zValidator } from '@hono/zod-validator'
+import { Hono } from 'hono'
+import { HonoEnv } from 'load-context'
+import cookieSessionStorage from 'utils/session.server'
+import { z } from 'zod'
+
+const app = new Hono<HonoEnv>()
+
+app.get(
+  '/',
+  zValidator(
+    'query',
+    z.object({
+      continue_to: z.string().optional(),
+    }),
+  ),
+  async c => {
+    const { continue_to } = c.req.valid('query')
+
+    const { getSession, commitSession } = cookieSessionStorage(c.env)
+    const session = await getSession(c.req.raw.headers.get('Cookie'))
+    session.flash('continue_to', continue_to || '/')
+    c.header('Set-Cookie', await commitSession(session))
+
+    // @sor4chi デザインたのんだ
+    const responseHtml = `
+<html lang="ja">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>ログイン</title>
+</head>
+<body>
+  <h1>ログイン</h1>
+  <p>ログインしてください</p>
+  <a href="/login/github">GitHub でログイン</a>
+</body>
+</html>
+`
+    return c.html(responseHtml)
+  },
+)
+
+app.all('/', async c => {
+  return c.text('method not allowed', 405)
+})
+
+export default app
diff --git a/webapp/api/oauth/callback.ts b/webapp/api/oauth/callback.ts
index eeec72d..9b46038 100644
--- a/webapp/api/oauth/callback.ts
+++ b/webapp/api/oauth/callback.ts
@@ -4,6 +4,7 @@ import { token, tokenScope } from 'db/schema'
 import { Hono } from 'hono'
 import { HonoEnv } from 'load-context'
 import { validateAuthToken } from 'utils/auth-token.server'
+import { binaryToBase64 } from 'utils/convert-bin-base64'
 import cookieSessionStorage from 'utils/session.server'
 import { z } from 'zod'
 
@@ -135,13 +136,11 @@ app.post(
       })
 
     // code (240bit = 8bit * 30) を生成
-    const code = btoa(
-      String.fromCharCode(...crypto.getRandomValues(new Uint8Array(30))),
-    )
+    const code = binaryToBase64(crypto.getRandomValues(new Uint8Array(30)))
 
     // access token (312bit = 8bit * 39) を生成
-    const accessToken = btoa(
-      String.fromCharCode(...crypto.getRandomValues(new Uint8Array(39))),
+    const accessToken = binaryToBase64(
+      crypto.getRandomValues(new Uint8Array(39)),
     )
 
     // DB に格納
diff --git a/webapp/utils/convert-bin-base64.ts b/webapp/utils/convert-bin-base64.ts
new file mode 100644
index 0000000..0ba82e5
--- /dev/null
+++ b/webapp/utils/convert-bin-base64.ts
@@ -0,0 +1,5 @@
+export const base64ToBinary = (base64: string) =>
+  Uint8Array.from(atob(base64), c => c.charCodeAt(0))
+
+export const binaryToBase64 = (bin: Uint8Array) =>
+  btoa(String.fromCharCode(...bin))