PURPOSE: To detect a body to be detected positively by preventing the effect of the regular reflection from the surface of the body to be detected or a transparent body covering the body to be detected.

CONSTITUTION: A light projecting element 1 projects light through a light projecting lens 3 and a polarization filter 4. The polarization filter 4 converts the passing light into the (p) polarized light along the planar direction of the incident plane. A photodetector 2 outputs the received light signal in the received state of the light. The light projecting axis and the light receiving axis are set at twice the Brewster angle of water. At this time, the reflectivity of the (p) polarized light with the water is zero. Therefore, the light from the light projecting element 1 which is reflected from a body to be detected 7 is received without the effect of the regular reflection from the water 6 with the photodetector.