An optical pickup which uses a gradient multiple-division type phase difference grating (3) to divide a laser beam from a semiconductor laser (1) into a main beam (30) and sub-beams (31, 32), and uses an object lens (5) to condense them on the track (61) of an optical disk (6). A reflected light is allowed to reflect off a beam splitter (4) via the object lens and is guided to optical detectors (8A-C) by a condensing lens (7). Accordingly, in a tracking error signal detecting method using the push-pull signals of the main beam and sub-beams, offsets produced by an object lens shift and a disk tilt can be cancelled at low costs without lowering the utilization efficiency of light.
