PURPOSE: To precisely determine the position of displacement zero position of the stylus of a probe.

CONSTITUTION: Even after the initial contact of a stylus with a workpiece, the movement of a probe is continued by a prescribed distance, and the probe is thereafter reversely moved. In a plurality of points of time during this reverse movement, the output values 21 of a probe measuring device and the output values 22 of a machine measuring device are simultaneously stored in a memory 26. The most suitable straight line passing a plurality of the recorded output values is calculated by a function generator 27, and the output value of the machine measuring device for the displacement zero point of the stylus is calculated by performing the extrapolation of the straight line by an extrapolating equipment 28, and outputted.