PURPOSE: To easily restore a state to the one before change only by designating a plane desired to restore and to improve workability by designating the plane that becomes the target of restoration of a block desired to restore, deciding whether or not the restoration of a designated plane can be feasible, displaying a temporary shape of the block after restoration when it is feasible, and executing restoration processing.

CONSTITUTION: Firstly, the plane that becomes the target of the restoration of the block desired to restore is designated in step S1. Concretely, the plane is designated by instructing two comprising lines of the plane. At this time, it is checked whether or not the plane is composed of instructed two lines. Then, it is decided whether or not the restoration of the designated plane can be feasible in step S2. Then, the constituent of the block after restoration is decided, and the temporary shape after restoration is displayed in step S3. Following that, the restoration processing is executed in step S4. It is executed based on a result in which the feasibility/infeasibility of the restoration processing in step S2 is decided and a display result for the temporary shape after restoration in step S3 on the content of the restoration processing.