There are provided an image generation system and information storage medium which can generate an image of a simple object with reduced processing load, the brightness of the simple object being variable according to the amount of light sent from a light source and received by the simple object. The image generation system comprises a light-source simple processing section (114) for performing a light-source simple processing relative to the simple object. The light-source simple processing section (114) performs computation for information relating to at least one of the brightness and color on primitive surfaces constructing the simple object, based on an angle difference between a line-of-sight vector of a virtual camera and a light vector from the light source. The primitive surfaces are set to have first and second color information, and information relating to the color of the primitive surfaces is computed by interpolation computation based the first and second color information and the information of at least one of the brightness and color of one of the primitive surfaces.