There is provided a reflective scanning optical system, which includes a light source unit for emitting a beam, a polygon mirror, and a single f optical element. Magnification m_z of the f optical element in an auxiliary scanning direction is set to satisfy: -4m_z-2 . . . (1). The light source unit is set so that a central axis of the beam incident upon the polygon mirror from the light source unit will be in a plane orthogonal to the rotation axis of the polygon mirror and out of an effective scan range of the beam dynamically deflected by the polygon mirror in regard to a main scanning direction. Each reflecting surface of the polygon mirror and the scan target surface are optically conjugate with each other in regard to the auxiliary scanning direction. The beam incident upon the first surface of the f optical element and the beam emerging from the first surface are separated from each other in the auxiliary scanning direction.