diff --git a/src/frames/IMainFrame.java b/src/frames/IMainFrame.java
index 678a2b1..6cf26ab 100644
--- a/src/frames/IMainFrame.java
+++ b/src/frames/IMainFrame.java
@@ -1,10 +1,8 @@
 package frames;
 
-import javax.swing.JPanel;
-
 // -------------------------------------------------------------------------
 //
 public interface IMainFrame {
-	void showMainPanel(JPanel nowPanel);
-	void showHistoryPanel(JPanel nowPanel, int index);
+	void showMainPanel();
+	void showHistoryPanel(int index);
 }
diff --git a/src/frames/MainFrame.java b/src/frames/MainFrame.java
index 25199bd..be02c9c 100644
--- a/src/frames/MainFrame.java
+++ b/src/frames/MainFrame.java
@@ -3,7 +3,6 @@
 import java.awt.BorderLayout;
 
 import javax.swing.JFrame;
-import javax.swing.JPanel;
 
 import panels.MainPanel;
 import panels.ShowHistoryPanel;
@@ -12,7 +11,7 @@
 //-------------------------------------------------------------------------
 // 
 public class MainFrame extends JFrame implements IMainFrame {
-	
+
 	// model
 	private CustomersModel model;
 
@@ -32,7 +31,7 @@
 
 		// initialize model
 		model = new CustomersModel();
-		
+
 		// initialize Panels
 		mainPanel = new MainPanel(this, model);
 		historyPanel = new ShowHistoryPanel(this, model);
@@ -43,18 +42,18 @@
 
 		mainPanel.setVisible(true);
 		this.add(mainPanel, BorderLayout.CENTER);
-		
+
 		// adjust window size
 		this.pack();
 	}
 
 	// -------------------------------------------------------------------------
-	// 
-	public void showMainPanel(JPanel nowPanel) {
+	//
+	public void showMainPanel() {
 		getContentPane().removeAll();
-		
+
 		mainPanel.updateTableByTransition();
-		
+
 		add(mainPanel);
 		validate();
 		repaint();
@@ -62,12 +61,12 @@
 
 	// -------------------------------------------------------------------------
 	//
-	public void showHistoryPanel(JPanel nowPanel, int index) {
+	public void showHistoryPanel(int index) {
 		getContentPane().removeAll();
-		
+
 		historyPanel.setVisible(true);
-		historyPanel.updateListModelByIndex(index);		
-		
+		historyPanel.updateListModelByIndex(index);
+
 		add(historyPanel);
 		validate();
 		repaint();
diff --git a/src/panels/MainPanel.java b/src/panels/MainPanel.java
index 862a32a..4dcf153 100644
--- a/src/panels/MainPanel.java
+++ b/src/panels/MainPanel.java
@@ -105,8 +105,8 @@
 	// private
 	// -------------------------------------------------------------------------
 	//
-	private void transitionToHistory(IMainFrame mainFrame, int index) {
-		mainFrame.showHistoryPanel(this, index);
+	private void transitionToHistory(int index) {
+		mainFrame.showHistoryPanel(index);
 	}
 
 	// -------------------------------------------------------------------------
@@ -159,7 +159,7 @@
 		//
 		public void actionPerformed(ActionEvent e) {
 			if (-1 < panelTable.getSelectedRow())
-				transitionToHistory(mainFrame, panelTable.getSelectedRow());
+				transitionToHistory(panelTable.getSelectedRow());
 		}
 	}
 }
\ No newline at end of file
diff --git a/src/panels/ShowHistoryPanel.java b/src/panels/ShowHistoryPanel.java
index 628fb45..2dee09e 100644
--- a/src/panels/ShowHistoryPanel.java
+++ b/src/panels/ShowHistoryPanel.java
@@ -109,8 +109,8 @@
 	// private
 	// -------------------------------------------------------------------------
 	//
-	private void transitionToMain(IMainFrame mainFrame) {
-		mainFrame.showMainPanel(this);
+	private void transitionToMain() {
+		mainFrame.showMainPanel();
 	}
 
 	// -------------------------------------------------------------------------
@@ -123,7 +123,7 @@
 		// -------------------------------------------------------------------------
 		//
 		public void actionPerformed(ActionEvent e) {
-			transitionToMain(mainFrame);
+			transitionToMain();
 		}
 	}