An optical device having a sub-wavelength grating formed in a specified region is disclosed that is able to prevent wave front degradation accompanying a phase difference of a polarized light beam passing through the optical device. The optical device includes a circular-belt-like region where the sub-wavelength diffraction grating is formed, and a center portion where the sub-wavelength diffraction grating is not formed. A vertically polarized light beam used for operations on a blue-light optical recording medium A has a phase difference in the sub-wavelength diffraction grating to be an integral multiple of 2π and hence is transmitted through the sub-wavelength diffraction grating. A horizontally polarized light beam used for operations on a blue-light optical recording medium is diffracted by the sub-wavelength diffraction grating. The light path length L1 of the light beam passing through the circular-belt-like region is the same as that of the light beam passing through the center portion without the sub-wavelength grating.