Abstract: This method of forming a polygon image comprises the steps of obtaining a plurality of polygons having normal line data as apex data and constituting a model; sorting the model constituted by the plurality of polygons into polygons of a first color part and polygons of a second color part by boundary lines consisting of the direction of a light source and normal lines of the model; pasting up a first mono-color texture on the sorted polygons having the first color part, and pasting up a second mono-color texture on the sorted polygons having the second color part; and dividing the polygons intersecting the boundary lines along the boundary lines, pasting up the first mono-color texture on the polygons belonging to the first color part out of the divided polygons, and pasting up the second mono-color texture on the polygons belonging to the second color part.