A multibeam optical system that employs a multiline laser source emitting a laser beam at predetermined wavelengths, such as, for example, at wavelengths 488.0 nm and 514.5 nm, and a diffractive beam-dividing element having diffractive grating to divide the incident beam into nine diffracted beams. A design wavelength &lgr;M of the diffraction grating, at which the light quantity of the incident beam is equally distributed among the plurality of diffracted beams, is determined so as to satisfy the condition 488.0 nm