Even if a return beam from a work to be measured does not agree with the center of a two-piece sensor, a correction value is calculated according to a voltage difference between two sensors of the two-piece sensor if the voltage difference is within a neighborhood range. The correction value is added to an actual position of an objective lens in an up-down direction, to calculate a movement amount of the objective lens up to a focused state (a state in which the return beam agrees with the center of the two-piece sensor).