A scanning optical system includes: a light source for emitting a light flux with a wavelength λ; a polygon mirror comprising a plurality of reflection surfaces for deflecting the light flux emitted from the light source for scanning the scanning surface; a light-converging lens group in a two-element structure. The light-converging lens group is arranged between the polygon mirror and the scanning surface and forms a light spot on the scanning surface. In the scanning optical system, the wavelength λ is not larger than 600 nm, and the scanning optical system satisfies a conditional expression relating to a number of the reflection surfaces of the polygon mirror, a scanning width of the scanning optical system, and a focal length of the light-converging lens group in a main scanning direction.