diff --git a/apps/server/src/middlewares/user-auth.ts b/apps/server/src/middlewares/user-auth.ts
index 9ae7e2cc..317b2aa4 100644
--- a/apps/server/src/middlewares/user-auth.ts
+++ b/apps/server/src/middlewares/user-auth.ts
@@ -2,13 +2,13 @@ import type { Middleware } from 'koa';
 import { users } from '../models';
 
 export const middlewareAuthorizeUser: Middleware = async (ctx, next) => {
-  const userId = ctx.state.userId as number;
+  const userId = ctx.state.userId as number | undefined;
 
   const user = await users.get(userId);
   if (!user) {
     ctx.status = 401;
     ctx.body = {
-      errorCode: 'user_not_found'
+      errorCode: 'unauthorized'
     };
     return;
   }
diff --git a/apps/server/src/models/user.ts b/apps/server/src/models/user.ts
index c108c831..a9ad32d4 100644
--- a/apps/server/src/models/user.ts
+++ b/apps/server/src/models/user.ts
@@ -48,6 +48,9 @@ export const Users = (prismaUser: PrismaClient['user']) =>
       if (account) {
         account = account.toLowerCase();
       }
+      if (!id && !account) {
+        return undefined;
+      }
 
       const user = await prismaUser.findUnique({
         where: {
diff --git a/apps/web/locales/en.json b/apps/web/locales/en.json
index 51b71a52..2cc641d2 100644
--- a/apps/web/locales/en.json
+++ b/apps/web/locales/en.json
@@ -7,6 +7,7 @@
   "navbar.logout": "Logout",
   "auth.toast.login": "Successfully logged in",
   "auth.toast.logout": "Successfully logged out",
+  "auth.toast.invalid-domain": "Invalid domain. The domain must be specified as \"example.com\" without any slashes.",
   "toast.api-error.title": "An error has occurred",
   "api-error.unauthorized": "Authentication failed: trying to re-login...",
   "api-error.invalid_request": "Internal error: Invalid request. Please contact the administrator for more information.",
diff --git a/apps/web/locales/ja.json b/apps/web/locales/ja.json
index 39c39f2a..a000f52d 100644
--- a/apps/web/locales/ja.json
+++ b/apps/web/locales/ja.json
@@ -7,6 +7,7 @@
   "navbar.logout": "ログアウト",
   "auth.toast.login": "ログインしました",
   "auth.toast.logout": "ログアウトしました",
+  "auth.toast.invalid-domain": "無効なドメインです。ドメインは「example.com」のように、スラッシュなどは含めずに指定してください。",
   "toast.api-error.title": "エラーが発生しました",
   "api-error.unauthorized": "認証に失敗しました: 再ログインしています...",
   "api-error.invalid_request": "内部エラー: 不正なリクエストです。詳しくは管理者にお問い合わせください",
diff --git a/apps/web/organisms/navbar/login-modal.tsx b/apps/web/organisms/navbar/login-modal.tsx
index 369ef082..9db11250 100644
--- a/apps/web/organisms/navbar/login-modal.tsx
+++ b/apps/web/organisms/navbar/login-modal.tsx
@@ -16,8 +16,10 @@ import {
   Link,
   useDisclosure,
   Spinner,
-  Collapse
+  Collapse,
+  useToast
 } from '@chakra-ui/react';
+import isValidDomain from 'is-valid-domain';
 import { ExternalLinkIcon } from '@chakra-ui/icons';
 import { FormattedMessage, useIntl } from 'react-intl';
 import { getDocsUrl } from '~/utils/constants';
@@ -48,6 +50,7 @@ const placeholderDomain: Record<ServerType, string> = {
 export const LoginModal: FC<Props> = ({ isOpen, onClose }) => {
   const { signIn } = useAuth();
   const intl = useIntl();
+  const toast = useToast();
   const {
     isOpen: isOpeningSignIn,
     onOpen: onOpenSignIn,
@@ -66,6 +69,14 @@ export const LoginModal: FC<Props> = ({ isOpen, onClose }) => {
         setHasNotDomain(true);
         return;
       }
+      if (!isValidDomain(domain)) {
+        toast({
+          title: intl.formatMessage({ id: 'auth.toast.invalid-domain' }),
+          status: 'error',
+          isClosable: true
+        });
+        return;
+      }
 
       onOpenSignIn();
       if (serverType === ServerType.Mastodon) {
@@ -75,7 +86,7 @@ export const LoginModal: FC<Props> = ({ isOpen, onClose }) => {
       }
       onClose();
     })();
-  }, [onOpenSignIn, onClose, signIn, domain, serverType]);
+  }, [signIn, domain, onOpenSignIn, serverType, onClose, toast, intl]);
 
   useEffect(() => {
     if (!isOpen) {
@@ -103,6 +114,12 @@ export const LoginModal: FC<Props> = ({ isOpen, onClose }) => {
     }
   }, [domain]);
 
+  useEffect(() => {
+    setServerType(undefined);
+    setDomain('');
+    setHasNotDomain(false);
+  }, [isOpen]);
+
   return (
     <Modal isOpen={isOpen} onClose={onClose} isCentered>
       <ModalOverlay />