<p>An optical scanning device comprising a light-shielding plate (3) that shields a reflection light occurring when a scanning line emitted from a light source passes through a scanning lens (1), is reflected off a rotary polygon mirror (2), then passes through the scanning lens (1) again, and is reflected off the scanning lens (1) toward the opposite side of the rotary polygon mirror (2) across the scanning lens (1), wherein the light-shielding plate (3) shields a light at a position satisfying the following expression when the distance between the rotary polygon mirror (2) and the scanning lens (1) is L<sub</p>
