PURPOSE:To obtain a low cost, miniaturization and non-adjustment by processing a signal with digital filter though the signal is conventionally processed with an analog filter. CONSTITUTION:An error signal detecting part 3 to detect disk fluctuating quan tity X1, which is a signal to show the dislocation of an optical disk from a reference position, and fluctuating quantity DELTAX between a disk and a lens, which is the difference of optical system following quantity to be a control signal to move an objective lens, and to output an error output to correspond to the detected fluctuating quantity DELTAX between the disk and the lens, an A/D converter 4 to A/D converter the error output, a digital filter 5 to filter the output of the A/D converter 4, and a control circuit 6 to output the optical system following quantity based on the output of the digital filter, are provided. Thus, since the digital filter is used instead of the conventional analog filter, the capacitor of large capacity is not needed and an integrating circuit can be easily obtained. Then, the miniaturization and the non-adjustment can be obtained.
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