There is provided a scanning optical system for emitting a plurality of beams scanning in a main scanning direction. The scanning optical system includes a first and a second light emitting devices each of which has a linear array of light emitting points. The first and second light emitting devices are positioned such that the linear array of light emitting points of each of the first and second light emitting devices is aligned in parallel with an auxiliary scanning direction and that the beam spots formed by the plurality of beams emitted by the first light emitting device and the beam spots formed by the plurality of beams emitted by the second light emitting devices are alternately arranged on a scan target surface in the auxiliary scanning direction.