An optical pickup apparatus comprising a diffraction device in which a laser beam from a light source is diffracted by the diffraction device to produce a main beam and two sub-beams which are focused on a recording medium, the beams reflected from the recording medium aare diffracted by the diffraction device to be directed to at least one photodetector, whereby a tracking error signal can be detected from output signals of the photodetector by the three-spot method that is so highly reliable that an offset never arises in the tracking error signal even when the optical axis of the optical system is displaced from a given position.