diff --git a/websocket-sharp/HttpResponse.cs b/websocket-sharp/HttpResponse.cs
index 111a0d92..831b7278 100644
--- a/websocket-sharp/HttpResponse.cs
+++ b/websocket-sharp/HttpResponse.cs
@@ -104,11 +104,9 @@ namespace WebSocketSharp
 
     public bool IsWebSocketResponse {
       get {
-        var headers = Headers;
-        return ProtocolVersion > HttpVersion.Version10 &&
-               _code == "101" &&
-               headers.Contains ("Upgrade", "websocket") &&
-               headers.Contains ("Connection", "Upgrade");
+        return ProtocolVersion > HttpVersion.Version10
+               && _code == "101"
+               && Headers.Upgrades ("websocket");
       }
     }