A virtual plane surface PL is divided into a plurality of square regions such that the closer distance to a virtual camera VP a square region is located at, the smaller areas the square region is divided into. Distance information 41 indicative of distances of respective vertices composing each of the square regions from the virtual plane surface PL is read from the internal main memory 11e. Further, coordinate points of positions, which are distanced from respective vertices, which compose polygonal shape regions included in each of the square regions, by distances indicated by the read distance information in a direction perpendicular to the virtual plane surface PL, are used as polygon vertices, which define polygons, whereby the polygons corresponding to the curved surface SF are generated. From each of the square regions, substantially a common number of polygons are generated. In this manner, appropriate polygon information of the curved surface SF which is capable of securing a drawing quality can be generated.