1. A three-dimensional image processing apparatus for creating a shadow image on the outer surface of a 3D model using a shadow model formed by a plurality of polygons, comprising: shadow model storage means for storing at least coordinates of vertices of the shadow model, polygon sorting means for sorting the polygons forming the shadow model into front-facing polygons facing in directions toward a viewpoint of a virtual camera and back-facing polygons facing in directions opposite from the viewpoint of the virtual camera, and shadow image creating means for creating the shadow image in pixels which are pixels corresponding to the front-facing polygons minus those corresponding to the back-facing polygons.