PURPOSE: To provide a light detector fixing mechanism in which the deviation of the position of an attaching member is not generated, at the time of fixing a light detector attaching member whose position adjustment is completed with a screw.

CONSTITUTION: An attaching member 1 is inserted into an oval shape insertion port 9 provided at a substrate 2, and screws 5a and 5b are screwed through a plate-shaped member 8 between the side face of the substrate and the insertion port 9 into screw holes 4a and 4b. Then, the plate-shaped member 8 is deformed, and the attaching member 1 is pressurized and fixed. Thus, the fixing operation time of the light detector can be considerably reduced.