In an optical scanning device, a rotational axis of a first rotary polygon mirror and a rotational axis of a second rotary polygon mirror are inclined toward the same side with respect to a direction perpendicular to a reference plane inclusive of each of center axes of a first member to be scanned and a second member to be scanned, as viewed in directions of the center axes of the first member to be scanned and the second member to be scanned. As a consequence, a width in an arrangement direction of a plurality of rotary polygon mirrors can be reduced, thus achieving miniaturization without degrading a quality of an image in an image forming apparatus.