A signal, which is derived from the relationship between the lens shift amount of an objective lens and the amount of the positional-deviation, of a received light beam with respect to a light-receiving plane, which occurs in the tangential direction on a photodetector, is corrected by an offset caused by asymmetry in the intensity distribution of reflected light; a multiplication value is obtained by multiplying the corrected signal by a predetermined constant; based on a focus error signal obtained by subtracting the calculated value from the calculation expression according to the conventional astigmatism method, the focusing servo control is performed.