Light emitted from a laser is separated into a main beam, first sub beams, and second sub beams. The distribution intensity of each first sub beams is the same as that of the main beam. The distribution intensity of each second sub beam is different from that of the main beam. The sum of the focusing error signals relating to the main beam and first sub beams is a final focusing error signal. The difference between the tracking error signal relating to the main beam and the tracking error signal of each first sub beam is a final tracking error signal. The deviation of the thickness of the substrate of the disk and the radial tilt thereof are detected on the basis of the deviation of the zero crossing points of the focusing error signal and the tracking error signal relating to the main beam and each second sub beam.