PURPOSE: To efficiently execute the conversion processing from a wire frame model to a surface model by executing a face kind decision, based on set information of an edge line for holding a face component obtained in a model conversion process from the surface model to the wire frame model.

CONSTITUTION: In a CAD system, a solid model file 1, a shape model converting part 2, a wire frame model file 6, an eddy line set data part 3, a face kind decision processing part 4, and a face kind eddy line data file 5 are provided. In the conversion process to a wire frame model, a set of eddy lines for constituting the prescribed face is extracted, extracted eddy line set information is stored in an eddy line set data part 3, and the face kind decision processing part 4 reads out the eddy line set information from the eddy line set data part 3, decides the face kind constituted of the eddy line set, and writes it in the face kind eddy line data file 5. In such a way, the conversion processing from the wire frame model to the surface model can be attained efficiently.