In accordance with a loop open/close control signal, an analog switch 60 can close or open a loop which consists of a voltage controlled oscillator 1, a variable frequency divider 2, a phase comparator 3, a first loop filter 451, the analog switch 60, and a second loop filter 452. In order to reduce the change of frequency caused when the open loop state is set immediately after the output frequency is changed, the second loop filter 452 uses a capacitor which shows properties of a small change of the capacitance in response to an applied voltage and a small hysteresis. In order to reduce the change of frequency caused when the open loop state is set immediately after the output frequency is changed, furthermore, a second diode one terminal of which is grounded is connected in reverse parallel to a first diode switch which switches the output oscillation frequency ranges of the voltage controlled oscillator.