PURPOSE: To omit the process of position coordination of a photo-sensor with a specimen even when a probe is replaced and provide sensibility for displacement of the probe with good accuracy.

CONSTITUTION: A photo-sensor 12 is installed in a plane approx. perpendicular to the optical axis of the reflected light from the reflex surface 4 of a probe 2 and provided with a number of light receiving elements which are oriented within the plane in a certain direction. An adjusting means T decides a reference position which divides the photo-sensor 12 into the first light receiving part and the second light receiving part, wherein the reference position is capable of being changed to any desired position in the receiving element oriented direction, and light information sensing means 20, 21, 22, 23 sense the first light information and second light information on the basis of photoelectric signals given by the first and second receiving parts. The first and the second light information are compared with each other by a comparing means 24, and a signal in accordance with the displacement of the reflex surface 4 is emitted.