PURPOSE: To narrow down the effective noise band width of a phase locked loop to an input signal with an extremely low level and to perform demodulation with a high S/N ratio by setting the control gain of the automatic gain controller at the front stage of the phase locked loop below a set value to the input signal with a level lower than the threshold level of the phase locked loop.

CONSTITUTION: The phase difference between an FM input signal and the output of a voltage-controlled oscillator 7 is detected by a phase detector 4. Its detection output is passed to a loop filter 5 to remove an out-band noise and fed back to the voltage-controlled oscillator 7 to control its frequency. Consequently, the oscillation frequency of the voltage-controlled oscillator 7 coincides with the freqency of the FM input signal and its phase state is locked. When the input signal level drops below the threshold level of the phase locked loop, the control gain of the automatic gain controller connected to the front stage of the phase locked loop decreases below the set value correspondingly, and consequently the effective noise band width of the phase locked loop is decreased to lower the threshold level, demodulating the FM input signal with the extremely low level at a high S/N ratio.