PURPOSE: To improve drawing performance remarkably by drawing data on a frame buffer when all windows are present on the frame buffer.

CONSTITUTION: When a window operation command is inputted from an input device 1, a window control processing means 5 divides rectangularly the drawing area of a window on the frame buffer 12 which holds a picture element image on a display device 13, and a rectangle list is generated on a storage device 8 to perform display control. Then when a drawing control flag 10 is set on the storage device 8, a window saving processing means 6 and part or the whole of the window enters a nondisplay state, window display data on the frame buffer 12 are saved in a logical image plane which holds the image data and a window damage processing means 7 refers to the rectangle list to transfer the contents of the logical image plane to the frame buffer 12. Consequently, the drawing performance is improved extremely.