Abstract: A graphic data processing system including an entering unit for entering a two-dimensional polygon, a copying unit for copying the two-dimensional polygon entered by the entering unit to another location separated from the original location by a prescribed distance, and a connecting unit for connecting vertexes of the two-dimensional polygon entered by the entering unit and corresponding vertexes of the two-dimensional polygon copied by the copying unit, whereby to form a prismatic figure. Further, the graphic data processing system may include a vertex-entering order judging unit for judging the order of the vertexes of the two-dimensional polygon entered by the entering unit, and a shade processing unit for processing shades of the prismatic figure in response to a result of the judgement made by the vertex-entering order judging unit.