Abstract: In order to accurately detect a tracking error signal, the position of a sub-beam Bs1 with a lower shading rate is located at a position ahead of the position of a main beam Bm in the recording direction of an optical disk, and the position of a sub-beam Bs2 with a higher shading rate is located at a position behind the position of the main beam Bm in the recording direction of the optical disk. Therefore, the unequal amount of optical information created by the different shading rates is counterbalanced by the fact that the portions of the optical disk where the sub-beams Bs1 and Bs2 are directed have different reflective rates. Accordingly, the amount of optical information from each sub-beam received by the tracking error signal photo detector is made equal, and the tracking error signal can be accurately detected.