PURPOSE: To securely prevent malfunction and to hold phase noise characteristics excellent by holding the relation between a local frequency and a main oscillation frequency in an invariably optimum state not only in a stationary state, but also even when frequency switching is performed.

CONSTITUTION: A control voltage generating circuit 40 generates a control voltage V'MIX for switching which is a certain value lower than a control voltage VLO for local oscillation. Then a control voltage switching circuit 28 supplies the control voltage V'MIX to VCO 21 when the difference between a control voltage VMIX outputted from the loop filter 26 of a PLL circuit 20 for main oscillation and the control voltage V'MIX for switching is less than a specific voltage value which is previously set. When the difference between the control voltage VMIX and the control voltage V'MIX for switching is larger than the specific voltage value, on the other hand, the control voltage V'MIX for switching is supplied to the VCO 21 instead of the control voltage VMIX.