diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index cccc0b1b..c6b65630 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -2234,6 +2234,7 @@ namespace WebSocketSharp
     ///   <paramref name="content"/> is <see langword="null"/>.
     ///   </para>
     /// </exception>
+    [Obsolete ("This method will be removed.")]
     public static void WriteContent (
       this HttpListenerResponse response, byte[] content
     )