PURPOSE: To prevent occurrence of the malfunction caused by the external noise by applying the pulses synchronizing with the synchronous pulses so that the rectifier circuit may operate only for the period of the synchronous signals extracted by the differential amplifier.

CONSTITUTION: In synchronous signal detecting/smoothing circuit 20 which converts the synchronous signals into the control signals to stop the variation, the composite signals are applied to transistor TR1 of differntial amplifier 28 with installation of synchronous signal extraction level deciding variable resistor VR1 at TR2. And the synchrouns pulses extracted at amplifier 28 are applied to rectifier circuit 29. circuit 29 possesses the synchronous pulse to the synchronous signal as the power supply, and the output voltage of circuit 29 is suppled to comparator 25 to be compared with the reference voltage E. Then the output of comparator 25 is applied to the control unit. Thus, pulse P2 synchronizing with the synchronous pulse is applied so that circuit 29 may operate only for the period of the synchronous signals extracted at amplifier 28 in order to prevent occurrence of the mal function caused by the external noise.