An image displaying apparatus for a computer selectively executing a full-screen display or a frame display on a monitor screen is disclosed. The apparatus includes a main screen generating unit generating a predetermined main screen, a full-screen displaying unit displaying the main screen in a full-screen display, a mouse operation determining unit detecting a moving operation of the mouse to determine whether or not the moving operation is a predetermined first movement, a full-screen display detecting unit detecting whether or not the main screen is in the full-screen display, and a composite displaying unit displaying a predetermined sub-screen on the screen and the main screen downscaled so that the whole main screen is displayed on the remaining range of the screen when the mouse operation determining unit determines, during the full-screen display, that the moving operation is the first movement.