An optical device includes: a light source that emits a light beam; a deflector that deflects the light beam in a main-scanning direction; and a scanning/image-forming optical system that includes a scanning lens that causes the light beam deflected by the deflector to converge to a scanned surface to form an image on the scanned surface. The scanning lens has refractive index gradient. At least one surface of the scanning lens is a tilt-decentered surface that has a tilt angle, which depends on a position in the main-scanning direction, in a sub-scanning direction. The tilt angle is set so as to compensate for variation, which results from the refractive index gradient, in direction of the light beam in the sub-scanning direction.