PURPOSE: To enable a semiconductor photodetector to be set maximal in relative sensitivity and protected against erroneous operation caused by adventitions light other than light rays emitted from a light emitting device even if the semiconductor photodetector deviates slightly in an optical axis from the light emitting device.

CONSTITUTION: Incident light S1 and S2 focused by a lens 5 are made to impinge on a light receiving plane 1a of a photodetective chip 1, whereby electrical signals are generated in a semiconductor photodetector corresponding to the intensity of the incident lights S1 and S2. The lens 5 is formed into nearly an elliptical hemisphere, and the focal point P of the lens 5 is set on the light receiving plane 1a. By this setup, a semiconductor photodetector of this constitution can be less affected by spherical aberration or the like.