PURPOSE: To effectively use the screen by switching to the plural window mode, when it is necessary to access simultaneously plural windows.

CONSTITUTION: In the plural screen mode 2, a window switching button 7 is displayed on a part of the window as shown in 3-2, 3-3, and 3-4. When this button 7 is clicked by a mouse, etc., a window switching pull-down menu 6 is displayed as shot in 3-3. and in this menu 6, a table of the windows which are opened at present is displayed. When a window 5-3 to be switched is selected from therein by clicking it by the mouse, etc., it is switched to the target window 5-3 as shown in 3-4. In the end, at the time of returning to a plural window mode 1 from the plural screen mode 2, when mode switching 4-2 is generated by a special kind key again, it returns to the plural window mode again as shown in 3-5.