PURPOSE: To execute clamp tracing effectively by eliminating repeated processings applied to a surface which has once been processed in case the work undergoes tracing processing after the contour of a model while a tool is controlled so that it does not proceed to under the set clamp limitation.

CONSTITUTION: The coordinate XN where a tool transfers into the tool traceing motion, and coordinate XP as going out of the clamp tracing motion, are stored. To update the level LCZ of the clump limit, the limits LXN, LXP in the tracing range are altered on the basis of these coordinate values XN, XP. By this process, the tracing range can be narrowed automatically at the time of updating the clamp level.