PURPOSE: To easily change a carrier frequency, by using a line the coupling degree of which is not greatly fluctuated against the carrier frequency change, as a means coupling a varactor diode to a dielectric resonator.

CONSTITUTION: One end of a line 9 connected to an anode of a varactor diode 5 is short-circuited as high frequency by a through-capacitor 8 to a ground potential. Since a distance l3 from a close range between the line 9 and a dielectric resonator 1 to the short-circuit point and a distance l4 to a diode 5 give less effect on the degree of coupling, even if a carrier frequency is changed, the degree of coupling can not be changed much. Since a DC bias voltage and a modulation signal are applied to the diode 5 via the capacitor 8 and the line 9 from an input terminal 7, the diode is independently of the carrier frequency. Through the constitution like this, in changing the carrier frequency, the resonance frequency of a subresonance circuit 6 for correction of modulation linearity has only to be adjusted so as to obtain the best modulation linearity due to the double tuning characteristics, allowing to easily change the carrier frequency.