PURPOSE: To measure the displacement without receiving the effect of the outside or the reflection factor, by projecting the light of a light emitting diode which is controlled through a temperature compensating circuit to an object to be measured through an optical fiber and detecting the reflected light by a photodetector and an amplifier which is capable of giving the gain control in accordance with the reflection factor of the light.

CONSTITUTION: A current is flowed to a light emitting diode 3 from a power supply E through a temperature compensating circuit 4, and the light of the diode 3 is projected to an object to be measured through an optical fiber 1. Then the reflected light is detected by a photodetector 5 via an optical fiber 2, and the output of the detector 5 is supplied to an amplifier 7 which is capable of giving the gain control in accordance with the reflection factor as well as to a current-voltage converting circuit 6. With this output, the displacement is detected for the object to be measured. In such way, the displacement can be measured without-receiving the effect of the outside or the reflection factor.