PURPOSE: To cover a wide frequency range, by preparing plural voltage control oscillators and switching them.

CONSTITUTION: The phase of an output signal f0 of a voltage control oscillator 13-1 selected by a selecting circuit 14 is compared with that of an input signal fi. A voltage corresponding to their phase difference is outputted from a phase comparator 11. The voltage control oscillator 13-1 is controlled by this output voltage to set the phase difference between signals f0 and fi to zero. When signals f0 and fi are not phase locked with each other, a beat signal corresponding to the differential frequency between two signals is generated. If this beat signal is detected, the selecting circuit 14 energizes a next voltage control oscillator 13-2 and stops the operation of the voltage control oscillator 13-1. Voltage control oscillators are switched in this manner to cover a wide frequency range.