PURPOSE: To measure the minute displacement of an object to be measured highly accurately by combining two semiconductor lasers and a polarizing beam splitter, and measuring the frequency when the light outputs from the reverse surfaces of the lasers are varied by displacement.

CONSTITUTION: Lenses 16 and 17 are provided between semiconductor lasers 11 and 12 and the polarizing beam splitter 13. The light beams from the reverse surfaces of the lasers 11 and 12 are received by light receiving elements 14 and 15. Laser light beams L1 and L2 from the lasers 11 and 12 are synthesized by the beam splitter 13 and irradiated on the object. The reflected light beam is recombined to the lasers 11 and 12 through the splitter 13 and the lenses 16 and 17. The laser outputs from the reverse sides are detected by the light receiving elements 14 and 15. When the object is displaced, wavering of the light output occurs. Since the frequency of the laser output is inversely proportional to the distance to the object, the frequency when the wavering occurs is measured, and the distance to the object is measured by a specified expression. Therefore the displacement of the body can be detected highly accurately.