An optical scanning device includes a plurality of light sources configured to emit a plurality of light beams, respectively; an optical deflector having a deflection-reflection surface that deflects and reflects the light beams, each of the light beams being incident on the deflection-reflection surface at an oblique incident angle, the oblique incident angles of the light beams being different from one another; and a flat glass arranged to be tilted at a tilt angle with respect to the deflection-reflection surface in a sub-scanning direction, the light beams being incident on the deflection-reflection surface via the flat glass, the tilt angle being smaller than a largest one of the oblique incident angles of the light beams. The light beams deflected and reflected by the deflection-reflection surface are incident on a plurality of scanned surfaces via the flat glass, respectively.