diff --git a/endpoints/slack.py b/endpoints/slack.py
index fbb731f..e9f7bf4 100644
--- a/endpoints/slack.py
+++ b/endpoints/slack.py
@@ -212,7 +212,11 @@ def _load_cached_history(self, channel: str, thread_ts: str):
             data["messages"] = messages
             data["last_cleanup"] = now
             try:
-                self.session.storage.set(key, json.dumps(data).encode("utf-8"))
+                if not messages:
+                    # メッセージが空ならキー自体を削除
+                    self.session.storage.delete(key)
+                else:
+                    self.session.storage.set(key, json.dumps(data).encode("utf-8"))
             except Exception:
                 pass
         return messages