PURPOSE: To eliminate the generation of spuriousness due to the leakage of a reference frequency without extremely aggravating noise near signals, to miniaturize circuit constitution and to lower a cost in the phase locked loop circuit of a frequency synthesizer.

CONSTITUTION: In this phase locked loop circuit of the frequency synthesizer, a control voltage 17 is prepared from the fundamental frequency F-REF 5 component of a 1/F-REF 12 by a one-shot multivibrator 10, a phase locked loop is turned off only during a period t13 when the F-REF 5 leaks by an S/H circuit 9 inserted between a VCO 1 and a phase detector 3 and a loop filter 4, a voltage immediately before turning it off is held during the period, it is turned on in a sample state in the other periods and a VCO 1 circuit is operated.