PURPOSE: To reduce a channel switching time and to make the synthesizer stable against a temperature change by connecting a 2nd varactor diode to the oscillator of a voltage controlled oscillator(VCO), and applying a DC voltage corrected based on a voltage data read corresponding to the channel data.

CONSTITUTION: The capacitance of a 1st varactor diode 6 and that of a 2nd varactor diode 10 are set to the capacitance corresponding to the applied voltage and a remarkable change in the frequency due to channel changeover is realized by the capacitance change in the 2nd varactor diode 10 and the 1st varactor diode 6 is used only for fine adjustment of the switched frequency. That is, a change in the DC voltage applied to the 1st varactor diode 6 is a narrow range for the phase matching in the process of the lock of the PLL, then the channel changeover is attained in a short time without being affected with the time constant of a lag lead filter 4.