diff --git a/kis-api/main.py b/kis-api/main.py
index 251f521..1827553 100644
--- a/kis-api/main.py
+++ b/kis-api/main.py
@@ -44,10 +44,19 @@ scheduler = AsyncIOScheduler(timezone="Asia/Seoul")
 class KiwoomToken:
     token: str = ""
     expires_at: datetime = datetime.min
+    _lock = asyncio.Lock()
 
     async def get(self, client: httpx.AsyncClient) -> str:
         if self.token and datetime.now() < self.expires_at:
             return self.token
+        # 동시 갱신 방지: 락 안에서 재확인(double-check) — 키움은 앱키당 토큰 1개만
+        # 유효(재발급 시 이전 토큰 무효화)하므로 thundering-herd 시 무효 토큰을 붙들게 됨
+        async with self._lock:
+            if self.token and datetime.now() < self.expires_at:
+                return self.token
+            return await self._refresh(client)
+
+    async def _refresh(self, client: httpx.AsyncClient) -> str:
         resp = await client.post(
             f"{KIWOOM_BASE_URL}/oauth2/token",
             json={"grant_type": "client_credentials",