An optical head includes a laser source (11) irradiating the laser beam
having a wavelength not longer than 500 nm, a collimator lens (13) for
collimating the laser beam to the parallel rays, an optical beam shaping
system (14, 15) that adjusts the cross sectional shape of the beam from
the collimator lens, and an objective lens (16) that converges the beam
on an optical information medium. The optical head has a relationship
between a focal length f.sub.CL of the collimator lens and a
magnification M of beam shaping satisfies the following
equation:.gtoreq..times..function..times. ##EQU00001## in which C is a
constant in (.lamda.mm).sup.1/2, A.sub.so is a maximum astigmatism per
displacement of the collimator lens from the focal position, and n is a
refractive index of a member of optical beam shaping system.