PURPOSE: To calculate the face wobbling value in a short time by transmitting the holding clear signals to the 1st and 2nd peak holding circuits with a measurement start signal transmitted to a measuring gate generation circuit respectively and at the same time controlling these circuits.

CONSTITUTION: A selector 10 selects and outputs a maximum or minimum value holding signal (j) or (k) with a selection signal 1 received from a control part 12. An A/D converter 11 applies the A/D conversion to the output of the selector 10 with a conversion start signal (m) received from the part 12 and outputs a conversion end signal (n) and the conversion data (o). Then the error of the stored data (o) is multiplied by the sensitivity of a focus actuator for calculation of the face wobbling value. Thus the focus servo is applied to an optical disk via an optical head and the face wobbling value is measured with the drive signal of the focus actuator. Then measuring time is shortened for the face wobbling value.