In an optical scanning device, an optical deflector includes a base plate, a rotor having an axis of rotation orthogonal to the base plate, and a polygon mirror fixed to the rotor. The polygon mirror has a plurality of reflecting surfaces tilted at a predetermined angle with respect to the axis of rotation. Protrusions protruding from a bottom surface of a frame have bearing surfaces at respective distal ends thereof; and the bearing surfaces are in contact with the base plate. The bottom surface includes a first region on which a first protrusion is disposed, and a second region on which a second protrusion is disposed, and a distance between the first region and a scanning plane containing optical axes in cross sections taken along a sub scanning direction of a scanning lens is different from a distance between the second region and the scanning plane.