The optical scanning device includes a polygon mirror, one or more planar mirrors for reflecting the beam light so as to be led to a photosensitive drum, and a pressing member for pressing a back surface of a final planar mirror. The final planar mirror has a glass plate for allowing the beam light to pass therethrough and a reflection layer arranged on an opposite surface of the glass plate. The beam light passes through the glass plate and is reflected at the opposite surface side, so that a scanning line is curved on a surface to be scanned in a convex shape to one side in a sub-scanning direction. The final planar mirror is pressed by the pressing member so as to be curved, so that the scanning line on the surface to be scanned is adjustable to a flat shape.