PURPOSE: To perform tracking control with high accuracy by always performing the tracking control basing on an optical axis as a reference and correcting an offset of a galvanomirror.

CONSTITUTION: The galvanomirror 19 for changing the direction of a light beam for irradiation of a disk 7 is turned by a driving circuit 26. A track deviation signal is obtained from an output of a photodetector 12 for detecting a deviation of an irradiating position, with the light beam from a track on the disk. A rotary angle at the time when the galvanomirror 19 is turned, is detected by a photosensor 21, and an offset due to the turning is canceled by this turning angle, so that a differential amplifier 13 for correcting the track deviation signal is equipped. At the time of retreaving a track on the disk 7, an output of a phase compensation circuit 24, where an added output of an output of a low-pass filter based on the output of the photodetector 12 and an output of a DC-cut filter based on the output of the photosensor 21 is phase- compensated, is negatively fed back to a driving input of the galvanomirror 19, and even in case of disturbance, the galvanomirror 19 is locked to make the center of a light spot coincide with the center of an optical axis of an optical system.