diff --git a/websocket-sharp/Ext.cs b/websocket-sharp/Ext.cs
index 31737971..b5ec9335 100644
--- a/websocket-sharp/Ext.cs
+++ b/websocket-sharp/Ext.cs
@@ -238,6 +238,13 @@ namespace WebSocketSharp
              : null;
     }
 
+    internal static string CheckIfCanStart (this ServerState state)
+    {
+      return state == ServerState.Start || state == ServerState.ShuttingDown
+             ? "This operation has already been done."
+             : null;
+    }
+
     internal static string CheckIfStart (this ServerState state)
     {
       return state == ServerState.Ready
diff --git a/websocket-sharp/Server/HttpServer.cs b/websocket-sharp/Server/HttpServer.cs
index 7f3bdae6..06a01efb 100644
--- a/websocket-sharp/Server/HttpServer.cs
+++ b/websocket-sharp/Server/HttpServer.cs
@@ -892,7 +892,7 @@ namespace WebSocketSharp.Server
     public void Start ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfStartable () ?? checkIfCertificateExists ();
+        var msg = _state.CheckIfCanStart () ?? checkIfCertificateExists ();
         if (msg != null) {
           _logger.Error (msg);
           return;
diff --git a/websocket-sharp/Server/WebSocketServer.cs b/websocket-sharp/Server/WebSocketServer.cs
index a6155157..93610dc6 100644
--- a/websocket-sharp/Server/WebSocketServer.cs
+++ b/websocket-sharp/Server/WebSocketServer.cs
@@ -795,7 +795,7 @@ namespace WebSocketSharp.Server
     public void Start ()
     {
       lock (_sync) {
-        var msg = _state.CheckIfStartable () ?? checkIfCertificateExists ();
+        var msg = _state.CheckIfCanStart () ?? checkIfCertificateExists ();
         if (msg != null) {
           _logger.Error (msg);
           return;