Focus servo control based on a focal point error signal by a conventional astigmatism method has caused an error in a focal point error signal by a deviation in a received light beam position on the light reception surface of a photodetector and by offset caused by the asymmetrical intensity distribution of a return light, making impossible a correct focus servo operation. [MEANS OF SOLVING PROBLEMS] A signal, deduced from the relation between the positional deviation between a light reception surface and a received light beam that occurs in a tangential direction on a photodetector (12) and the shift amount of an object lens (7), is corrected by offset caused by the asymmetrical intensity distribution of a return light to determine a total value obtained by adding a specified constant tothe corrected value, and focus servo control is performed based on a focal point error signal obtained by subtracting this computed value from a computing expression by a conventional astigmatism method.