An optical scanning device includes a light source unit, a deflector, and a scanning lens. The deflector reflects first and second light beams emitted from the light source such that the beams scan a scanned surface in main scanning direction. The scanning lens is disposed between the deflector and the scanned surface, includes incident and emission surfaces, and focuses the first and second light beams on the scanned surface. At least one of incident and emission surfaces is an optical refractive surface in which first and second refractive surfaces through which the first and second light beams pass respectively are arranged in alignment in the sub scanning direction. An interval between a first generatrix of the first refractive surface and a second generatrix of the second refractive surface in the sub scanning direction increases from a center portion toward end portions of the scanning lens in the main scanning direction.