According to the present invention, highly accurate adjustment of the angles and positions of optical elements constituting an optical scanning unit can be easily performed. The optical scanning unit comprises: a first mirror that reflects the light beams emitted from Y, C and M laser diodes; a second mirror that reflects the light beam emitted from a K laser diode and the light beams reflected by the first mirror; a cylindrical lens that acts on the light beams reflected by the second mirror; and a third mirror that reflects the light beams emitted from the cylindrical lens toward a polygon mirror. The second mirror is provided with an adjusting mechanism that can adjust the light beams in the main scanning direction, and the third mirror is provided with an adjusting mechanism that can adjust the light beams in the sub-scanning direction.