PURPOSE: To grasp and verify the shape of loop patterns by tracing patterns divided at branch points from a start point to an end point and deciding an invalid loop pattern in accordance with the existence of a pattern connected to the branch point.

CONSTITUTION: A graphic pattern is divided at all branch points (a) to (d) and the divided patterns 1 to 5 are stored in tables (a) to (c). Two optional branch points, e.g. (a), (d) are set up as the start point (a) and end point (d) of a loop pattern, routes from the start point (a) to the end point (d) are traced, flags indicating traced patterns out of the patterns 1, 3, 2, 4 arrived at the end point (d) can be stored in the tables twice, and when an untraced pattern 5 connected to the traced routes is not left in the tables, a valid loop pattern is decided. Thus, the shape of all the loop patterns 1 to 5 can be grasped and verified.