A scanning optical system, includes a mirror unit having a first mirror surface and a second mirror surface which incline to a rotation axis; and a light projecting system having a light source. A light flux emitted from the light source is reflected on the first mirror surface of the mirror unit, thereafter, reflected on the second mirror surface, and then, projected so as to scan in a main scanning direction onto an object in accordance with rotation of the mirror unit. The light flux emitted from the second mirror surface becomes a plurality of spot lights on the object side, and the plurality of spot lights are arranged along a direction intersecting with the main scanning direction.