In order to correct a track zero-crossing signal TZC generated from a signal detected by an optical head of an optical disk system, an expected edge pulse and a window signal are generated at the position at which the appearance of the edge of the track zero-crossing signal is expected. Among the edge pulses of the actual track zero-crossing signal, only those that exist inside a window are allowed to pass through. When the edge pulses do not exist within the window, the expected pulse is inserted so as to generate a corrected edge pulse, and a corrected track zero-crossing signal is generated from this corrected edge pulse.