diff --git a/linebot_app.py b/linebot_app.py
index 0969181..d9be596 100755
--- a/linebot_app.py
+++ b/linebot_app.py
@@ -105,7 +105,7 @@ def callback():
 def message_text(event):
 
     log_file = open(log_file_name, "a")
-    log_file.write("user_id: " + event.source.user_id + ", time: " + event.timestamp + ", message:" + event.message.text)
+    log_file.write("user_id: " + event.source.user_id + ", time: " + str(event.timestamp) + ", message:" + event.message.text)
     # User Session
     # create session if none exist
     if event.source.user_id not in user_sessions.keys():