PURPOSE: To open and close a window wherein reference information is displayed without increasing the workload of a CPU so much when the reference information is projected on the monitor of a computer together with main information.

CONSTITUTION: The storage capacity of a video random access memory is set larger than the capacity of image data required for a display of one screen of the monitor, the image data of the main information are written in a specific area 51 wherein image data of one screen normally specified by the read address counter of the video random access memory are written, and the image data of the reference information, etc., are written in areas 52-54 except the specific area 51 at all times; and the image data of one screen are read out by changing the head address of the address signal that the read address counter outputs to change images displayed on the monitor.