PURPOSE: To prevent loss of measurement of a distance produced when feed is moved from clamp tracer to model copying.

CONSTITUTION: A distance L between optical distance sensors 5a and 5b and a model 6 is measured while the optical distance sensors 5a and 5b are moved relatively at a clamp copying speed along a clamp limit LCZ set vertically on a table placed on the model 6. When the measured value L becomes smaller than a preset comparison value L0, it is considered that the optical distance sensors 5a and 5b nears a switchover point P4 between clamp copying and model copying. Then a relative feed rate of the optical distance sensors 5a and 5b is reduced to prevent a relative follow-up delay of a tracer head and also prevent loss of measurement of a distance.