To provide an optical scanning device and an image forming apparatus capable of shifting a resonance frequency that an optical box vibrates to a high frequency range, suppressing local vibrations and evading density irregularities on a photoreceptor.

For this optical scanning device, an optical box houses a light source for emitting a laser beam, a polygon mirror for reflecting the laser beam from the light source, a polygon mirror motor for rotationally driving the polygon mirror, an image forming lens for image-forming a scanning laser beam generated by being reflected by the polygon mirror driven by the polygon mirror motor and a reflection mirror for reflecting the scanning laser beam which receives the action of the image forming lens, and is provided with at least three attaching parts held by a case body. In the optical box, the thickness t of the attaching part held by the case body has the relation of 2×Tave≤t≤H/2 with the average thickness Tave of the optical box and the outer wall height H of the optical box.