A cathode ray tube (CRT) controller (6) reads out a predetermined picture from a frame memory (2) storing a picture to be processed, and generates a bit pattern with respect to a window memory (5), having a size corresponding to the frame memory (2) and storing a bit pattern which specifies the validity or invalidity of picture processing. The pattern output by the CRT controller (6) controls the processing of the frame memory (2), so that picture processing is executed solely with respect to picture data in an area of the frame memory (2) specified by the bit pattern.