PURPOSE: To prevent an occurrence of the disturbance noise to a TV or the like, by stopping the oscillation of an oscillating circuit in a conduction mode and securing an automatic start for the oscillation in a service interruption mode.

CONSTITUTION: No reference frequency f1 nor power source +B0 emerges in a service interruption mode, and the NPN transistor 36 is turned off due to an elimination of the base voltage. Then a charging starts to the capacitor C to give oscillation to the oscillating circuit 19. The output reference frequency f2 of the circuit 19 is divided into the frequency the same as the frequency f1 by a dividing circuit 18. On the other hand, the pulse RT produced by the outputs 1W3 of the circuit 18 is delivered after counted up to a prescribed value by the circuit 18. Thus the FF27 is set. The output (a) of FF27 is turned to 1, and the AND gates 25 and 24 of a switch circuit 14 are turned on and off respectively. Then the output of circuit 18 is applied to the time counting circuit 15. When the service interruption is recovered, the oscillation of circuit 19 is discontinued. And the FF27 is reset by the differential output LE of frequency f1, and the frequency f1 is delivered again from the circuit 14.