PURPOSE: To accurately execute the structural analysis of a graphic image by discriminating whether or not two line segments at a branch point in a diagram are formed in continuous structure.

CONSTITUTION: Assuming vectors in which a point Pi in a coordinate sequence of points is set as one terminal as PiPj1, PiPj2,..., PiPjn, the value of distance dimk between the point Pi and a virtual line segment PjmPjk is calculated for the line segment PiPjm and the line segment PiPik (where, it is m≠k), and calculated distance dimk is compared with a prescribed value (δ) set in advance, and it is discriminated whether or not the line segment is the one in which the PiPik and the line segment PiPjm are continued based on a comparison result. Therefore, it is possible to surely discriminate whether or not the tow line segments at the branch point Pi in the diagram are formed in the continuous structure. Thereby, reliability of results of the structural analysis of the graphic, etc., can be improved.