An original image is displayed on a display screen. Here, when a sub-screen input mode is set, a sub-screen that is obtained by reducing an entire screen displayed on the display screen is displayed on an image display portion. The user uses an electronic pen to select an optional input area within the sub-screen. An area corresponding to the selected input area is displayed on the entire screen. At this timer, the reduced image of the entire screen is switched to an image of the input area on the sub-screen. Then, the user uses the electronic pen to perform an editing work on the sub-screen. The sub-screen is set within reach of the user, and editing on an area out of reach on the entire screen can be also performed on the sub-screen.