Every other input pulse is extracted from an input signal containing alternate phase-advanced and phase-delayed pulses read out of a magnetic disk unit, a magnetic tape unit or the like. Output pulses in synchronism with the pulses thus extracted are produced. This extraction may be effected only during a predetermined length of time after staring application of the input pulses at the time of pull-in of a phase sync circuit. Whether phase synchronization is to be effected in accordance with advanced-phase or delayed-phase input pulses is determined from time to time as required.
