An optical element includes a diffraction grating by which both a first laser beam and a second laser beam are diffracted. A diffraction performance of the diffraction grating is expressed by an optical path difference function, a third spherical aberration SA3.sub.1 of the first laser beam and a third spherical aberration SA3.sub.2 of the second laser beam in a wave front aberration calculated by the optical path difference function satisfy the following conditional expressions: 0.005<|SA3.sub.1-SA3.sub.2|<0.015[.lamda.rms] SA3.sub.1.times.SA3.sub.2<0.