PURPOSE: To enable quick and easy positioning between an optical fiber and a photodetecting part by forming plural reflecting plates for positioning around the photodetecting part to be coupled to the optical fiber.

CONSTITUTION: When the light from a light source 9 propagates in an optical fiber 7 and is irradiated to a chip 1, the light reflected from a reflecting plate 4 for positioning returns in the fiber 7, is reflected by a beam splitter 8 and is detected by a detecting part 10 for the reflected light. The detection signal detected by the part 10 increases on approaching of the fiber 7 to a photodetecting part 2 and the intensity of the reflected light decreases sharply at the point where the center of the part 2 and the center of the fiber 7 coincide and therefore said point is easily detectable. The light is irradiated to the chip of the photodetector consisting of the semiconductor via the optical fiber in the above- mentioned way and the centers of the photodetecting part and the optical fibers are made easily coincident with each other by moving the chip while monitoring the intensity of the light reflected from the reflecting plate for positioning.