diff --git a/README-ja.md b/README-ja.md
index 0f003b2d..9afd5f92 100644
--- a/README-ja.md
+++ b/README-ja.md
@@ -1172,7 +1172,7 @@ def get_user(self, user_id):
     if user is None:
         user = db.query("SELECT * FROM users WHERE user_id = {0}", user_id)
         if user is not None:
-            key = "user.{0}".format(user_id)
+            key = f"user.{user_id}"
             cache.set(key, json.dumps(user))
     return user
 ```