This invention realizes a detection system which serves as a displacement measurement apparatus for optically detecting a relative position, improves the use efficiency of a light amount, and is resistant to mechanical variations. More specifically, in an optical sensor having a light source, a reflector for reflecting a divergent light beam from the light source, and a light-receiving portion for receiving a reflected light beam from the reflector. The reflector has an optical characteristic that a divergent light beam from the light source is wavefront-split into a plurality of beams and the respective split light beams are overlaid at a predetermined position.