PURPOSE: To switch two different frequencies at a high speed without changing a reference frequency.

CONSTITUTION: Mixers 23, 24 for frequency conversion are provided in a loop of a frequency synthesizer including a PLL oscillation circuit 21. Then the mixer 24 mixes an output fR±fIF of a voltage controlled oscillator in the PLL oscillation circuit 21 and a carrier fIF to implement frequency conversion. On the other hand, when a switch 22 is opened and no carrier fIF is given, the frequency fR±fIF passes. Moreover, the frequency fR and the carrier fIF are mixed by the mixer 23 to implement the frequency conversion and when no carrier fIF is given, the frequency fR±fIF passes. In this case, the ON/OFF of the carrier fIF fed to the mixers 23, 24 is implemented by the switch 22. Thus, the output frequency of the frequency synthesizer is switched at a high speed only with ON/OFF of the mixers 23, 24 without changing the reference frequency in the PLL loop.