PURPOSE: To extract all recessed areas of a solid model by executing a decision of labeling of irregularity, with respect to all edge lines for forming a polyhedron.
CONSTITUTION: An input part 1 sends out a data of a solid model, which is obtained from a CAD (Computer Aided Design), etc., to an edge line irregularity deciding part 2. A control part 3 controls the edge line irregularity deciding part 2, executes a decision of irregularity to one edge line, repeats an operation for sending the edge line to which a label has been given, and surface information to a fundamental data storage part 4, and executes labeling of irregularity to all the edge lines. A recessed area existence deciding part 5 retrieves the label of each edge line stored in the fundamental data storage part 4, and checks whether a recessed edge line exists or not. When existence of a recessed edge line 14 is extracted from one label, the extraction of this recessed edge line is informed to a recessed are extracting part 6. Based on this recessed edge line 14, the recessed area extracting part 6 searches a surface 10 and 11 for forming a recessed area from the surface information stored in the fundamental data storage part 4, and writes the extracted surface 10 and 11 to a recessed area storage part 7.