PURPOSE: To make assembling simple and an adjusting operation easy, by providing a shielding member which shields a deflected beam and a guide member which guides a beam to a detector, and constituting a holding member which holds the guide member and the shielding member to one body.

CONSTITUTION: A mirror 13 is provided in the part of a laser printer or the like before the part where a laser beam 12 enters the region of a photosensitive drum. A block 27 is fixed to the exit side of the mirror 13 and an optical fiber 22 is mounted thereto. A slit (shielding member) 27a is provided in one body to the block 27, and an edge 27c which is an end for interrupting the beam is formed roughly perpendicularly to the scanning direction of the beam 12. When a laser spot 19 crosses the edge 27c of the slit 27a, an output is generated in the photodetector 16 connected to the output end of the fiber 22, and the start timing for the input signal to a modulator for applying light information on the photosensitive drum is determined by the output signal thereof.