diff --git a/twspace_dl/main.py b/twspace_dl/main.py
index af367f2..57924c6 100644
--- a/twspace_dl/main.py
+++ b/twspace_dl/main.py
@@ -155,7 +155,7 @@ def merge(self) -> None:
             f"author={author}",
             "-metadata",
             f"episode_id={self.id}",
-            f"./{self.title}-{self.id}.aac",
+            f"./{self.title}-{self.id}.m4a",
         ]
         subprocess.run(command, check=True)
         logging.info("finished merging")