An optical scanning device has: a first optical system for converting a flux of light that has been emitted from a light source into another flux of light; a second optical system for focusing the flux of light that has been converted by the first optical system as a linear image elongated in the main scanning direction; deflecting means for reflecting and deflecting the flux of light that has left the second optical system; and a third optical system for guiding the flux of light that has been deflected by the deflecting means onto a surface to be scanned. The flux of light emitted from the light source is entered into a deflection surface of the deflecting means at a limited angle in the sub-scanning section and, by setting parameters in a manner that satisfies a given condition, the pitch unevenness can effectively be reduced in the optical systems.