PURPOSE: To surely mount a rotary polygon mirror, a laser light source and a synchronization detecting sensor on each prescribed position in a housing with high precision and by an easy work.

CONSTITUTION: As for a composite substrate 40, a motor substrate part 41 with a coil and a control circuit for a polygon motor 30, etc., mounted, a laser light source substrate part 42 with a laser light source 14 and a control circuit for the laser light source 14), etc., mounted and a synchronization detecting sensor substrate part 43 with the synchronization detecting sensor 18 and a control circuit for the synchronization detecting sensor 18, etc., are integrally molded. A holder is molded so that it may be brought into tight contact with the housing 44 without leaving a space between them by applying insert-molding, or outsert-molding.