PURPOSE: To avoid generation of biting-in by presuming the position of a quick change part in the next tracing pass by the use of the displacement amount from a tracer head while tracing is being performed, and sinking the tracing speed in the next tracing pass from a point a certain distance prior to the quick change part presumed.

CONSTITUTION: No.1 quick change part P1c on the model 1 surface is sensed by each shaft displacement amount obtained at the preceding tracing pass, and No.1 position in the specified plane of at least No.1 quick change part is stored in memory, and No.2 quick change part P2c continued to No.1 quick change part on the model surface is sensed using each shaft displacement position obtained in the tracing pass at this time. Then No.2 position in a specified plane of the No.2 quick change part is stored in memory, and in the next tracing pass the tracing speed is sunk in the division where the position in the specified plane is within a certain distance in front of and behind the intersection with the extension line passing No.1 position and No.2 position, and biting-in of a cutter in the work at the quick change part on the model is avoided.