PURPOSE: To display a video picture on screen data by converting the screen data, obtained by overwriting a video signal on the picture data by a graphic processor, from digital to analog, line by line, and displaying it on a CRT display device.

CONSTITUTION: The graphic display device is provided with a picture display memory (FM) 4 where the screen data for graphic display is plotted and a video buffer 9 to store a digitized video signal. Then the video signal is overwritten on the picture data by the graphic processor (GPU) 3 and the overwritten screen data is converted by the D/A converting circuit 5, line by line, into analog data, which is displayed on a CRT display device. Consequently, video images can be displayed on the CRT display device 1 on a graphic screen one over another in a window state by the simple and inexpensive circuit.