PURPOSE: To improve the efficiency of assembling work without necessity of inclining a photodetector and to improve the photoelectric conversion efficiency by providing a spherical lens having larger photodetecting area than the photodetecting unit of the photodetector between a light emitting element and the photodetector, directing the curved surface of the spherical lens toward the light emitting element to prevent the reflected light from being returned to the light emitting element.
CONSTITUTION: When a current is injected by a lead terminal 33 to a light emitting element 20, lights 22a, 22b are emitted. The light 22a emitted from the back surface is collected by a spherical lens 29 fixed to a photodetector 21 to be introduced to a photodetector 25. In this case, since the light 22a of the area larger than the surface area of the photodetector 25 can be input by the effect of the lens 29, the photoelectric conversion efficiency is improved, the reflected light 22c on the surface of the lens 29 is scattered so that the light returned to the element 20 is reduced. Accordingly it is not necessary to mount obliquely the photodetector 21 to improve the efficiency of the mounting.