PURPOSE: To avoid a mistake of demodulation when the synchronizing signal is dropped out by performing the demodulation by means of a subsynchronizing signal and at the same time to avoid the rotational disturbance of a motor when the synchronizing signal is dropped out by controlling the motor rotation in response to the output of comparison.

CONSTITUTION: A synchronizing signal detecting circuit 1 produces a synchronizing signal (a) of a frequency f1 with every detection of a synchronizing part. A differentiating circuit 2 produces fine differential pulses with the signal (a). At the same time, an oscillating circuit 3 generates an oscillating output (d) of a frequency f2 which is N times as high as the signal (a) and supplies it to the 1st frequency dividing circuit 4. Thus the circuit 4 divides the output (d) and transmits a subsynchronizing signal (e). The circuit 2 performs the matching of timing by resetting the circuit 4 with the timing of the synchronizing signal (b). As a result, the stable signal (e) is produced from the circuit 4 with the interpolation of the dropped-out synchronizing signal althrough the signal (b) has a dropout as long as the phase is once matched.