To improve the performance and further improve the operability when two screens are used as independent screens.

A memory actually stores the areas of dummy screens 43 and 44 which do not exit as a frame memory space for an original screen 41 and an extension screen 42. A flicker driver 9 informs an operating system 2 and a GDI(graphical device interface) 3 of the total size of the dummy screens 43 and 44 together with the original screen 41 and extension screen 42. Consequently, the parts of the original screen 41 and extension screen 42 which project from the dummy screens 43 and 44 are not transferred to the frame memory.