An objective lens (1) for an optical disk, which focuses a light beam from
a light source, is designed so that a third-order coma aberration
generated when the objective lens is inclined at a unit angle is larger
than a third-order coma aberration generated when the optical disk (2) is
inclined at the unit angle, mounted on an actuator for inclining the
objective lens according to an inclination amount of the optical disk,
and used. With this structure, it is possible to obtain an objective lens
for an optical disk that has a large numerical aperture and is easy to
manufacture and assemble, and in which the third-order coma aberration
generated when the optical disk surface is inclined owing to a warp or
the like can be corrected by small inclination of the objective lens, so
as to reduce a residual astigmatism, which is generated according to the
inclination amount, after the correction.