In an optical scanning device using two polygon mirrors, the present invention achieves both a simplification of the device and a reduction of vibrations. An optical scanning device is provided with a first optical system including a polygon mirror and a second optical system including a polygon mirror. The polygon mirror provided in the first optical system and the polygon mirror provided in the second optical system are configured such that they rotate in mutually opposite directions. Further, the optical scanning device is provided with one BD sensor for detecting laser beams that are scanned with the polygon mirrors. Slits with mutually different widths are arranged between the BD sensor and the respective polygon mirrors of the first and second optical systems.