In an image forming apparatus provided with an optical beam scanning apparatus according to the invention, an optical beam scanning apparatus of an overillumination scanning optical system includes a semiconductor laser device as a light source, a pre-deflection optical system, a polygon mirror, and a post-deflection optical system, with a width of the luminous flux made incident on the polygon mirror being wider than a width of one reflecting surface forming the polygon mirror, wherein at least two sheets of flat plate for transmitting the luminous flux scanned by the polygon mirror are provided in the post-deflection optical system. In accordance with an image forming apparatus provided with an optical beam scanning apparatus according to the invention, not only a wave front aberration on a photoconductive drum can be suitably corrected, but suitable beam diameter and beam profile can be obtained on the photoconductive drum.