PURPOSE: To enable a tracking correction control of a multibeam in simple constitution by rotating a diffraction grating based on a tracking error signal from a laser beam.

CONSTITUTION: An output of a sensor 6b for receiving reflected light from a main beam 15 is inputted to a differential amplifier 18, and a tracking error signal Push-Pull 1 (for controlling an objective lens) of a push-pull system is produced by this amplifier 18. The objective lens 2 is driven in the directions of T←→T' by this signal. Moreover, reflected light of either of side beams 14 and 16 is detected by either 6a and 6c of tracking sensors 6, and a tracking error signal Push-Pull 2 (for controlling the diffraction grating) of the push-pull system is produced by a differential amplifier 17. This signal is used as a control signal for the diffraction grating 7. That is, by rotating the diffraction grating 7 in the directions of θ←→θ' in order to make this signal zero, the center of a signal track is scanned by all of the three beams.