Diffraction grating, optical pickup device and optical disc apparatus

A pickup device splits and detects a light beam reflected from a disc with the use of a grating. The grating has first to six areas, wherein the first area and the second area, the third area and the fifth area, and the fourth area and the sixth area are arranged in point symmetry with respect to the center of the grating, respectively. The first area is interposed between the fourth area and the fifth area and the second area is interposed between the third area and the sixth area. Further, the centers of the first area and the second area are arranged to be spaced by a distance d in a direction perpendicular to the displacement direction of the optical pickup device. Even though the center of an optical disc is not located on a straight line in the displacement direction of the optical pickup device, objective lens can obtain a stable servo signal.