A half-wave plate is attached to a lens holder to integrally drive a collimator lens and the half-wave plate. When the lens holder is moved to a servo position, the half-wave plate is inserted in and retracted from an optical path of a laser beam. Therefore, the laser beam is formed in S-polarized light or P-polarized light with respect to a polarization beam splitter, and the laser beam is guided to a first objective lens or a second objective lens.