diff --git a/src/main/java/info/istlab/IoTP/Launcher.java b/src/main/java/info/istlab/IoTP/Launcher.java
index 5474bd6..aa42f36 100644
--- a/src/main/java/info/istlab/IoTP/Launcher.java
+++ b/src/main/java/info/istlab/IoTP/Launcher.java
@@ -144,7 +144,7 @@
         menu.add(mi);
 
         mi = new JMenuItem(isEnglish ? "MQTT Client" : "MQTTクライアント");
-        mi.addActionListener(ae -> MqttWindow.create() );
+        mi.addActionListener(ae -> MqttWindow.create().setVisible(true) );
         menu.add(mi);
 
         setJMenuBar(menuBar);
diff --git a/src/main/java/info/istlab/IoTP/MqttWindow.java b/src/main/java/info/istlab/IoTP/MqttWindow.java
index 02d0dac..15398f6 100644
--- a/src/main/java/info/istlab/IoTP/MqttWindow.java
+++ b/src/main/java/info/istlab/IoTP/MqttWindow.java
@@ -252,13 +252,15 @@
             subscriber = null;
             appendToJTA("Disconnected.\n");
             jta.setBackground(Color.gray);
+            MqttWindow.theMqttWindow = null;
         }
         connectB.setText("Connect");
-        MqttWindow.theMqttWindow = null;
         if (Launcher.theapp == null || !Launcher.theapp.isVisible())
             System.exit(0);
-        else
+        else {
+            setVisible(false);
             dispose();
+        }
     }
 
     @Override
diff --git a/src/main/java/info/istlab/IoTP/SerialWindow.java b/src/main/java/info/istlab/IoTP/SerialWindow.java
index dfb6e04..003f244 100644
--- a/src/main/java/info/istlab/IoTP/SerialWindow.java
+++ b/src/main/java/info/istlab/IoTP/SerialWindow.java
@@ -52,7 +52,7 @@
             SerialPort sp = splist_valid.get(0);
             App.serialName = sp.getSystemPortName();
         } else if (splist_valid.size()==0){
-            if (popup_if_zero) JOptionPane.showMessageDialog(null, "No available serial ports.");
+            if (popup_if_zero) JOptionPane.showMessageDialog(Launcher.theapp, "No available serial ports.");
         } else {
             SerialSelector.show(splist_valid.toArray(new SerialPort[]{}));
         }
@@ -72,7 +72,7 @@
             SerialWindow sw = new SerialWindow(sp);
             hash.put(sp.getSystemPortName(), sw);    
         } else if (splist_valid.size()==0){
-            JOptionPane.showMessageDialog(null, "No available serial ports.");
+            JOptionPane.showMessageDialog(Launcher.theapp, "No available serial ports.");
         } else {
             SerialSelector.show(splist_valid.toArray(new SerialPort[]{}));
         }
@@ -85,7 +85,7 @@
         sp.addDataListener(this);
         System.out.println(sp.getSystemPortName());
         if (!sp.openPort()) {
-            JOptionPane.showMessageDialog(null, "他のプログラムがシリアルポート "+sp.getSystemPortName()+" を使用しています。");
+            JOptionPane.showMessageDialog(Launcher.theapp, "他のプログラムがシリアルポート "+sp.getSystemPortName()+" を使用しています。");
             System.out.println("Unable to open the port.");
             return;
         }