An optical system for focusing a light beam emitted from a semiconductor laser is disclosed which comprises a lens of a numerical aperture NA satisfying a relation ##EQU1## (where .lambda. and .DELTA.Z indicate a laser wavelength of and an astigmatic focal distance in the semiconductor laser, respectively) to make the wave front aberration of a light beam emerging from the lens smaller than or equal to .lambda./4, thereby correcting astigmatism in the semiconductor laser.