An optical system has an expander lens and an objective lens. The following formula is satisfied: W1.sub.CM>W2.sub.CM, where W1.sub.CM is a coma aberration (.lamda.rms) of a converged light spot when an off-axial light flux of a wavelength .lamda. (nm) emitted so as to converge at a distance position from the optical axis comes into the objective lens, and W2.sub.CM is a coma aberration (.lamda.rms) of a converged spot when an off-axial light flux of the wavelength .lamda. (nm) emitted so as to converge at the distant position distant comes through the expander lens into the objective lens on a condition that the optical axis of the expander lens is arranged so as to conform with the optical axis of the objective lens.