Abstract: A diffraction grating comprises a birefractive sheet (11) having a corrugated surface with at least each groove filled with a mass (12) of a material of a refractive index which is substantially equal to one of ordinary and extraordinary indices of the birefractive sheet. The diffraction grating is effective in an optical head device when made to have a plurality of grating regions of different patterns of corrugations. In this event, a linearly polarized beam should be supplied to the diffraction grating with electric vectors directed parallel to those of ordinary and extraordinary components when the refractive index is substantially equal to the ordinary and extraordinary indices, respectively. Alternatively, such an optical head device may comprise a reflection grating in which reflective corrugations are substantially parallel in the respective grating regions. In this event, a linearly polarized beam is supplied from an optical source to the reflective corrugations as a p-polarized beam.