An optical pickup (20) projects a read laser beam onto an optical disk (40) rotated by a spindle motor (30) and receives the reflected beam. The laser beam emitted from a laser generating device (21) is transmitted through a liquid crystal panel (25) provided to correct the spherical aberration due to the thickness variation of a transmitting substrate of the optical disk and directed to an objective (26). A control circuit (50) varies a spherical aberration correct signal (SA) to take samples in a region where the variation of the output of an optical sensor (31) is large and to determine the position of the apex of an approximation curve by calculation as a correction value. Thus, the correction value for correcting the spherical aberration due to the thickness variation of the optical disk can be accurately determined in a short time.
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