PURPOSE: To shorten the switching time when two different levels of frequency are alternately generated.

CONSTITUTION: The output of a 1/N variable divider 3 is compared with the outpu of a reference frequency signal generator 1, and a phase comparator 4 generates the output in accordance with the phase difference of both outputs. Thus the control terminal voltage Vc of a voltage control oscillator 2 is controlled via a switch 8 and a loop filter 6 or a switch 9 and a loop filter 7. A control part 5 generates a division setting signal (a) of the divider 3 and the switch signals (g) and (h) of both switches 8 and 9. When the frequency f1 and f2 are generated by the oscillator 2 and then switched to each other, the part 5 controls both filters 6 and 7 so that these filters are alternately connected between the output (d) of the comparator 4 and a control terminal of the oscillator 2. Then the control voltage corresponding to the frequency charged to the capacitors 6a and 7a are kept even while the frequency of the other side is generated.