Transistors M11 and M12 are cross-coupled to each other so as to form a negative resistance circuit. First and second variable capacitor sections 10 and 20 are connected in parallel with an inductor circuit so as to form a parallel resonant circuit. A first reference voltage Vref1 and a control voltage VT1 corresponding to a carrier wave component are fed to both terminals, respectively, of each of variable capacitors VC11 and VC12 included in the first variable capacitor section 10. A second reference voltage Vref2 and a control voltage VT2 corresponding to a modulated wave component are fed to both terminals, respectively, of each of variable capacitors VC21 and VC22 included in the second variable capacitor section 20. ADC control section 30 includes a resistance R31 interposed between the second reference voltage Vref2 and a common connection point of resistances R21 and R22, and a capacitor C31 for feeding only an AC component included in the modulated wave component to the common connection point of the resistances R21 and R22. Thus, a DC voltage, on which the AC component included in the modulated wave component is superimposed, is changed from a certain DC bias to a DC bias of the second reference voltage Vref2.