PURPOSE: To prevent the inclusion of dust and stray light by providing a hole of a diameter larger than that of a hole which is closed in a bottom part and has a photodetector in the bottom part to said hole, and fitting an optical fiber plug therein in a manner as to abut on its level differing part.

CONSTITUTION: A fiber holder 2 is fixed by means of screws 3 to a substrate mounted with a photodetector 7. A fiber plug 6 is fitted into a recess 2B' of a diameter larger than that of the hole 2A of the holder 2, and its leading end is abutted on a level differing part. Thereby, the inclusion of dust and stray light is prevented.