A scanning optical system using a short-wavelength light of 500 nm or less uses a reflecting mirror having a higher absolute reflectivity and having reduced wavelength and angle dependences. Divergent ray of light emitted from a semiconductor laser is converted into an approximately parallel light beam by a collimator lens and the diameter of the light flux is reduced by an aperture before travel to a polygon mirror. The light beam from the polygon mirror passes through scanning lenses to form a small spot at any point in the entire scanning area. The semiconductor laser is a gallium nitride semiconductor laser having an oscillation wavelength of 408 nm. The polygon mirror has such a characteristic that, if the complex refractive index N of a metallic film contributing to a reflection characteristic of the reflecting mirror is defined as N(.lamda.)=n(.lamda.)-ik(.lamda.), then k(.lamda.)> {square root over ((-n(.lamda.).sup.2+18n(.lamda.)-1))}{square root over ((-n(.lamda.).sup.2+18n(.lamda.)-1))} is satisfied.