PURPOSE: To start or restart the oscillation by using a preset frequency locked to a frequency of a reference signal so as to start or restart the oscillation thereby keeping a new initial phase in response to a trigger signal inputted optionally.

CONSTITUTION: A phase locked loop (PLL) consists of a voltage controlled oscillator 1, the 1st frequency divider 2, an OR circuit 9, a phase detector 4, a loop filter 10, a reference signal oscillator 11, the 2nd frequency divider 3 and an OR circuit 8. A monostable multivibrator 5 outputs the 1st signal (h) stopping or restarting the voltage controlled oscillator 1. A D flip-flop circuit 7 outputs an output signal (r) when the phase between an output signal (i) of the voltage controlled oscillator 1 and a reference signal (e) has a prescribed relation. A feedback loop is cut off in response to the stop state of the voltage controlled oscillator 1 by a monostable multivibrator 12, a D flip-flop 6 and the OR circuits 8, 9, the output of the phase detector 4 is kept and the phase feedback loop cut off by the signal (r) is connected. Thus, a signal (j) operating the frequency dividers 2, 3 from the initial state is outputted.