PURPOSE: To attain frequency changeover with simple constitution by applying switching control to a frequency at which a negative resistance of a negative resistive element is maximized so as to be made correspondent with each resonance frequency of 1st and 2nd dielectric resonators.

CONSTITUTION: A prescribed DC bias voltage is applied to a gate G and a source S of a field effect transistor (TR) (FET) 30 via 1st and 2nd DC bias voltage terminals 40, 42. Then a control voltage is applied to a varactor diode 46 connecting to a source S of the FET 30 with a control voltage terminal 47. With the control voltage applied to the FET 30, frequencies causing a maximum negative resistance system return gain A correspond to resonance frequencies of 1st or 2nd dielectric resonator 34 or 35 when the negative resistance system is viewed from the gate G of the FET 30. For example, the frequency is subject to switching control to be 10GHz or 10.75GHz. Thus, the frequency is switched with simple constitution.