diff --git a/packages/backend/src/server/api/SignupApiService.ts b/packages/backend/src/server/api/SignupApiService.ts
index d2c4440116..c72420689f 100644
--- a/packages/backend/src/server/api/SignupApiService.ts
+++ b/packages/backend/src/server/api/SignupApiService.ts
@@ -136,7 +136,20 @@ export class SignupApiService {
 				return;
 			}
 
-			if (ticket.usedAt) {
+			// メアド認証が有効かつ有効期限が0以外に設定されている場合
+			if (instance.emailRequiredForSignup && instance.emailVerificationExpiresIn !== 0) {
+				// メアド認証済みならエラー
+				if (ticket.usedBy) {
+					reply.code(400);
+					return;
+				}
+
+				// 認証しておらず、まだ有効期限内ならエラー
+				if (ticket.usedAt && ticket.usedAt.getTime() + (instance.emailVerificationExpiresIn * 1000 * 60) > Date.now()) {
+					reply.code(400);
+					return;
+				}
+			} else if (ticket.usedAt) {
 				reply.code(400);
 				return;
 			}
@@ -222,8 +235,13 @@ export class SignupApiService {
 		const code = body['code'];
 
 		try {
+			const instance = await this.metaService.fetch(true);
 			const pendingUser = await this.userPendingsRepository.findOneByOrFail({ code });
 
+			if (instance.emailVerificationExpiresIn !== 0 && this.idService.parse(pendingUser.id).date.getTime() + (instance.emailVerificationExpiresIn * 1000 * 60) < Date.now()) {
+				throw new FastifyReplyError(400, 'EXPIRED');
+			}
+
 			const { account, secret } = await this.signupService.signup({
 				username: pendingUser.username,
 				passwordHash: pendingUser.password,