PURPOSE: To prevent the occurrence of damage of a probe head part by an obstacle, by providing an obstacle detecting blowing port, detecting the presence of the obstacle, and controlling the ascending movement of the probe.

CONSTITUTION: Air is blown out of a blowing port 2 at the tip of a probe 1. The probe 1 is moved up and down so that the air pressure in the probe 1 becomes constant. The coordinates in the up and down directions in three-dimensional coordinates are determined. An obstacle detecting blowing port 3 is provided at the side surface in the sending direction of the probe 1. An obstacle such as a crest part and the like 4a is detected by the blowing of air from the blowing port 3, and the ascending movement of the probe 1 is controlled. Therefore, damage of the probe head by the obstacle is prevented, and the sutomatic measurement of the three-dimensional coordinates without the occurrence of the damage to the probe head can be performed.