PURPOSE: To hold a signal phase-locked to a reference signal even when the reference signal is missing.

CONSTITUTION: A voltage controlled oscillator 1, a phase comparator 3 outputting a signal proportional to a phase difference between the output of the voltage controlled oscillator 1 and the reference signal and a low pass filter 4 are connected sequentially to form a closed circuit 10 so that the output frequency of the voltage controlled oscillator 1 is locked to the reference signal. The circuit 10 is provided with a detector 5 detecting the missing state of a reference signal and outputting a high level signal to set the output of the phase comparator 3 to a high impedance. Thus, when the reference signal is missing, the output of the phase comparator 3 is set to a high impedance, the control voltage of the voltage controlled oscillator 1 is held in a state before the reference signal is missing to obtain the output signal of the voltage controlled oscillator 1 phase-locked to the reference signal.