There is provided an optical scanning device and an image forming apparatus including the optical scanning device. The optical scanning device includes an optical box containing light source units, a rotating polygon mirror, and a motor. In order to prevent the entire optical box from being deformed by deformation occurring in the installation region due to heat of the driving unit, the optical box includes a connection region between a second sidewall and an installation region where a driving unit is installed. The optical box further includes regions provided next to the connection region and having heights different from each other. A concave portion is formed in the connection region.