diff --git a/src/main/java/info/istlab/ServerTester/MulticastReceiver.java b/src/main/java/info/istlab/ServerTester/MulticastReceiver.java
index 3ec27d7..7a7c5d1 100644
--- a/src/main/java/info/istlab/ServerTester/MulticastReceiver.java
+++ b/src/main/java/info/istlab/ServerTester/MulticastReceiver.java
@@ -7,7 +7,7 @@
 import java.net.SocketException;
 import java.util.Hashtable;
 
-public class MulticastReceiver implements Runnable{
+public class MulticastReceiver implements Runnable {
     Thread thread;
     MulticastSocket socket;
     MulticastSender sender;
@@ -30,29 +30,33 @@
         remoteHostsWindow = new RemoteHostsWindow(receivedData);
     }
 
-    public void run(){
-        while(thread != null){
+    public void run() {
+        while (thread != null) {
             byte[] buffer = new byte[1024];
             DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
             try {
                 socket.receive(packet);
-            } catch(SocketException sx) {
+            } catch (SocketException sx) {
                 thread = null;
             } catch (IOException e) {
                 e.printStackTrace();
             }
-            String addr = packet.getAddress().getHostAddress();
-            String message = new String(packet.getData(), 0, packet.getLength());
+            if (!socket.isClosed() && packet != null) {
+                String addr = packet.getAddress().getHostAddress();
+                String message = new String(packet.getData(), 0, packet.getLength());
 
-            receivedData.put(addr, message);
-            if (remoteHostsWindow != null) {
-                remoteHostsWindow.updateData(addr, message);
-                remoteHostsWindow.frame.pack();
+                receivedData.put(addr, message);
+
+                if (remoteHostsWindow != null) {
+                    remoteHostsWindow.updateData(addr, message);
+                    remoteHostsWindow.frame.pack();
+                }
             }
             // System.out.println(addr+"からの受信: " + message);
         }
     }
-    public void stop(){
+
+    public void stop() {
         socket.close();
         sender.stop();
         // thread = null;