To provide a tool route for gouging a part not being fine between patches and to repair a tear between the patches without a failure, by preparing a mending patch whose vertexes are the coordinates of both ends of an incomplete ridge line for constituting the tear between the patches and the coordinates drawn from the coordinates in Z direction by specified distances.

Patch data and ridge line data stored in a data storage part 11 are read to a processing part 10 and the incomplete right line for not connecting the patches with each other is detected from the patch data and the ridge line data in an incomplete ridge line detection part. Then, in a mending patch preparation part, the coordinates of both ends of the detected incomplete ridge line and the coordinates drawn from the coordinates in Z direction by a prescribed distances are calculated and the mending patch whose vertexes are the coordinates at four points is prepared. Then, after preparing the mending patch for all the incomplete ridge lines for constituting the tear between the patches, the respective sides of the mending patch are detected as ridge lines and the ridge line data including the ridge lines of the mending patch are prepared.