An optical scanning unit includes a rotatable multi-faceted mirror having a plurality of faces reflecting light flux emitted from a light source to scan a scanning area in a main scanning direction. A width of the light flux striking the rotatable multi-faceted mirror is smaller than a length of a face of the rotatable multi-faceted mirror. The entire of light flux striking the rotatable multi-faceted mirror is reflected at a first face when the light flux reflected by the rotatable multi-faceted mirror is directed to the center portion of the scanning area. A part of the light flux striking the rotatable multi-faceted mirror is reflected at the first face while the remaining of the light flux is reflected at a second face when the light flux reflected by the rotatable multi-faceted mirror is directed to a least one of the two end portions of the scanning area.