PURPOSE: To prevent the omission of track cross pulse at a Miller mark part during a seek and to precisely detect the number of passed tracks and a pulse cycle.

CONSTITUTION: When the output voltage of the photoelectric conversion element of an optical head 1 is inputted in a differential amplifier 2 and an adding amplifier 3, the track error signal and a sum signal are respectively obtained. At the Miller mark part, the sum signal level is increased, and the Miller mark detecting signal is made true by a comparator circuit 7. When the Miller mark detecting signal becomes true, a gain is increased by an amplifier 4, and when false, it is decreased. Consequently, only in the Miller mark part, signal amplitude is enlarged. Since the signal amplitude is made even, the output without pulse omission is obtained from a waveform shaping circuit 5.