PURPOSE: To stabilize an output signal against temperature change by setting the DC loop gain near the leading-in frequency of an oscillator to a value smaller than the DC loop gain in other frequency regions in the servo circuit of an AFC loop system.

CONSTITUTION: The input signal (frequency f1 and phase 1) from terminal 17 is supplied to phase detector 18 and is compared with a feedback signal (frequency f2 and phase 2) in phase. After the comparison error voltage is integrated by integrating circuit 19, voltage control oscillator 21a is controlled by signal V3 amplified by DC amplifier 20, and the output signal is not only output from terminal 22 but also fed back to detector 18. In this DC loop, a Zener diode and a resistance are connected in parallel between amplifier 20 and oscillator 21a, and transfer function F(ω) of circuit 19 is set to a prescribed value to set DC loop gain df/d near the leading-in frequency of the oscillator to the 1/20 or more of the DC loop gain of other frequency regions. Thus, a stable operation can be obtained against temperature change when this circuit is used for the gen-lock circuit of a TV.