PURPOSE: To measure the fine displacement and surface roughness of a body to be measured accurately without any influence of a noise of a laser light source, by detecting reflected light from the body to be measured through two two- split photodetectors.

CONSTITUTION: The reflected light from the reflecting body 6 is reflected by a polarizing prism 3 and split by a half-mirror 30 into two, which are made incident to prisms 31 and 32 and guided to split photodetecting sensors 33 and 34. When the surface of the reflecting body 6 is at the focal point of an objective 5, namely, when the reflected light is parallel light, the photo-detection surfaces of the split photodetecting sensors 33 and 34 are uniform in the amount of photodetection and when the reflected light is divergent or convergent light, the output of each photodetection surface varies. For this purpose, the sum of the outputs of the photodetection surfaces 33B and 34B is subtracted from the sum of the outputs of the photodetection surfaces 33A and 34A to detect the position of the reflecting body 6.