To provide an optical scanning device that scans a scanning surface with a light beam accurately and stably.

The optical scanning device includes: a light source unit; a cylindrical lens; a polygon mirror; a deflector side scanning lens; and an image face side scanning lens or the like. The light source unit has: a dust-proof cover 57 which covers a light source 11, a coupling lens 12, an aperture plate 13, a reflection mirror 14 for monitoring, a condensing lens 15 so as to prevent the attachment of dust thereto ; and a dust-proof member 55 and a dust-proof glass 56 to prevent the attachment of dust on the light receiving surface of a light receiving element 16. In this case, the variation in the light quantity of a luminous flux emitted from the light source 11 is stably and accurately detected, and the stable luminous flux is emitted by automatic power control (APC).