diff --git a/src/router/index.ts b/src/router/index.ts
index 95f6ef6..474be5e 100644
--- a/src/router/index.ts
+++ b/src/router/index.ts
@@ -162,12 +162,12 @@ const router = createRouter({
 router.beforeEach((to, from) => {
   // to: Route: 即將要進入的目標 路由物件
   // from: Route: 當前導航正要離開的路由
-  let isRemoveCookie = document.cookie.split(';').some(c => c.trim().startsWith('expires=Thu, 01 Jan 1970 00:00:00 UTC;')); // 是否登入，有到期日表示已登出。
 
-  // 當路由到 login 時，有登入要跳轉至home
+  // 當路由到 login 時，已登入則跳轉至 Files
   if (to.name === 'Login') {
-   if (isRemoveCookie) router.push({ name: 'Files' });
+    const isLoggedIn = document.cookie.split(';').some(c => c.trim().startsWith('isLuciaLoggedIn='));
+    if (isLoggedIn) return { name: 'Files' };
   }
- });
+});
 
 export default router;
diff --git a/tests/router/routerGuard.test.js b/tests/router/routerGuard.test.js
new file mode 100644
index 0000000..aa2b255
--- /dev/null
+++ b/tests/router/routerGuard.test.js
@@ -0,0 +1,39 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+
+describe('router beforeEach guard logic', () => {
+  beforeEach(() => {
+    // Clear cookies
+    document.cookie.split(';').forEach((c) => {
+      const name = c.split('=')[0].trim();
+      if (name) {
+        document.cookie = name + '=; Max-Age=-99999999; path=/';
+      }
+    });
+  });
+
+  // Simulate the guard logic from router/index.ts
+  function runGuard(to) {
+    const isLoggedIn = document.cookie
+      .split(';')
+      .some((c) => c.trim().startsWith('isLuciaLoggedIn='));
+
+    if (to.name === 'Login') {
+      if (isLoggedIn) return { name: 'Files' };
+    }
+    return undefined;
+  }
+
+  it('redirects logged-in user from Login to Files', () => {
+    document.cookie = 'isLuciaLoggedIn=true';
+    expect(runGuard({ name: 'Login' })).toEqual({ name: 'Files' });
+  });
+
+  it('allows unauthenticated user to visit Login', () => {
+    expect(runGuard({ name: 'Login' })).toBeUndefined();
+  });
+
+  it('does not interfere with non-Login routes', () => {
+    document.cookie = 'isLuciaLoggedIn=true';
+    expect(runGuard({ name: 'Files' })).toBeUndefined();
+  });
+});