An entire view volume in a virtual three-dimensional game space to be displayed on a display unit is divided into a front view volume closer to a view point and a back view volume away from the view point. In a first frame of two successive frames, polygons in the back view volume are rendered to generate a back image. In a second frame, with the back image being taken as a background, polygons in a front view volume are rendered to form an entire image. With this, in a portable game machine including a relatively small-sized three-dimensional image processing circuit, polygons more than those that can be generated in one frame by the three-dimensional image processing circuit can be displayed.