diff --git a/packages/backend/src/core/UserAuthService.ts b/packages/backend/src/core/UserAuthService.ts
index bdc27cbe8e..dc9104e8a2 100644
--- a/packages/backend/src/core/UserAuthService.ts
+++ b/packages/backend/src/core/UserAuthService.ts
@@ -4,17 +4,18 @@
  */
 
 import { Inject, Injectable } from '@nestjs/common';
-import { QueryFailedError } from 'typeorm';
+import * as Redis from 'ioredis';
 import * as OTPAuth from 'otpauth';
 import { DI } from '@/di-symbols.js';
 import type { MiUserProfile, UserProfilesRepository, UsersRepository } from '@/models/_.js';
 import { bindThis } from '@/decorators.js';
-import { isDuplicateKeyValueError } from '@/misc/is-duplicate-key-value-error.js';
-import type { MiLocalUser } from '@/models/User.js';
 
 @Injectable()
 export class UserAuthService {
 	constructor(
+		@Inject(DI.redis)
+		private redisClient: Redis.Redis,
+
 		@Inject(DI.usersRepository)
 		private usersRepository: UsersRepository,
 
@@ -30,16 +31,43 @@ export class UserAuthService {
 				twoFactorBackupSecret: profile.twoFactorBackupSecret.filter((secret) => secret !== token),
 			});
 		} else {
-			const delta = OTPAuth.TOTP.validate({
+			// 1. 判定に用いるタイムスタンプを固定
+			const now = Date.now();
+			const normalizedToken = token.trim();
+			const validationWindow = 1;
+			const timeStep = 30; // TOTPの周期（秒）
+
+			// 2. TOTPインスタンスを生成（設定を一元管理するため）
+			const totp = new OTPAuth.TOTP({
 				secret: OTPAuth.Secret.fromBase32(profile.twoFactorSecret!),
 				digits: 6,
-				token,
-				window: 5,
+				period: timeStep,
+			});
+
+			// 3. 固定したタイムスタンプを使って検証
+			const delta = totp.validate({
+				token: normalizedToken,
+				window: validationWindow,
+				timestamp: now,
 			});
 
 			if (delta === null) {
 				throw new Error('authentication failed');
 			}
+
+			// 4. totp.counter() を用い、同じタイムスタンプから基準ステップを取得
+			const currentStep = totp.counter({ timestamp: now });
+			const step = currentStep + delta;
+
+			const usedTokenRedisKey = `2fa:used:${profile.userId}:${step}`;
+
+			// 5. TTL（有効期限）の設定
+			const ttl = timeStep * (validationWindow * 2 + 1);
+			const setResult = await this.redisClient.set(usedTokenRedisKey, normalizedToken, 'EX', ttl, 'NX');
+
+			if (setResult === null) {
+				throw new Error('authentication failed');
+			}
 		}
 	}
 }