PURPOSE: To provide a fixing device for a photodetector capable of position adjustment only by moving a holding member by sticking/fixing the holding member on an optical base through a fixing member after moving the holding member for the photodetector and adjusting its position.

CONSTITUTION: A holder 40 holding a photodetector 24 is mounted on an adjusting tool not shown in the figure, the detector 24 is made to confront the recessed groove 41 of an optical base 21 and a reflected light beam R' from an optical disk is made incident on it. The output of the reflected beam R' is observed as an electric signal, the detector 24 is moved in parallel in three axial directions by the adjusting tool and positioned so as to obtain a prescribed waveform. Then, the both sides of the holder 40 are coated with adhesives 33, 33, the side part of the optical base 21 is coated with adhesives 35, 35 and the surfaces 34, 32 of the fixing member 31 are stuck. After the adhesives are hardened, the adjusting tool is removed. By such constitution, the fixing device for the photodetector capable of eliminating the shift of the photodetector at the time of fixation and the position adjustment of the photodetector is performed only by moving the holding member is obtained.