To allow a resolution to be changed by a single drawing command for each window instead of using different drawing commands in an application program depending on resolutions, in order to display a plurality of windows having different resolutions on a single display screen by use of a drawing facility for drawing a window specified by a drawing command from an application program with a resolution selected from a plurality of resolutions, first, windows to be drawn are registered in a list. The list is accessed in response to a drawing command from an application program to determine the resolution of a window to be drawn. The drawing facility is instructed to draw the window with the determined resolution. The image of the window drawn by the drawing facility is written into a frame buffer. The image is read from the frame buffer and displayed on a screen.