PURPOSE: To prevent the malfunction at the stationary action time by installing the impedance circuit which is varied by the output signal of the in-phase detector between the loop filter of the phase lock loop circuit and the phase comparator.

CONSTITUTION: Variable impedance circuit 20 is connected between phase comparator 1 which gives a phase comparison between the fixed input signal and the output of voltage control oscillator (VCO) 2 and low pass filter 3 which features a linear variation of the impedance by the output level of the comparator and then eliminates the high band component to generate the control voltage for VCO. Then the impedance of circuit 20 is reduced greatly to the high band signal component by turning off at the stationary action time inverter transistor Q2 of in-phase detector 5 which gives the output when the in-phase is obtained between the fixed input signal and the output of VCO and also turning on transistor Q1. Thus the loop gain can be decreased to reduce the leak component of the angle modulation signal, accordingly preventing the ON of the nonlinear element of the filter.