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By making the servo system insensitive to the spiked noise having high frequency components, a stable focus control and/or tracking control of an optical disk drive is realized. A digital signal processor is provided for sampling an output of an error detecting circuit for detecting an amount of deviation of control with a constant period to perform a phase compensation therefor and outputting a drive instruction value for the focus or track control on the basis of a signal sampled in a just preceding period when an error signal value sampled in a current period is larger than the error signal value sampled in the just preceding period.