PURPOSE: To further accelerate the processing speed for erasing or moving a window concerning a window display cache system for holding bit map data and accelerating plotting.

CONSTITUTION: The bit map data of the window extended by a window extending means 6 are stored in a cache area 5 and after the bit map data of a preceding image already displayed in a display area are saved into the cache area 5 by a bit map saving means 3, those data are written in a bit map area 2 by a bit map updating means 4. When erasing the displayed window, it is enough only to write the bit map data of the preceding image saved in the cache area 5 by using the bit map updating means 4. When displaying the erased window again, the bit map data of the window stored in the cache area 5 are utilized. A window control means 8 manages the display state of the window, judges an instruction from a GUI application 9 and controls the respective means.