PURPOSE: To easily adjust the amplitude of a track error signal to a standardized value without detecting the amplitude.

CONSTITUTION: The duty of track servo is measured by converting a track error signal TES obtained when the track servo is turned off into a pulse signal by using an upper and lower slice levels against the center set in the wind comparator of a duty measuring means 4. Then the amplitude of the signal TES is changed by controlling the gain of a variable gain amplifying means 5 by means of an amplitude controlling means 5 so that the measured duty can become a preset target value.