PURPOSE: To prevent oscillation at impedance mismatching with a device of the front stage by selecting a loss of a feedback circuit of an amplifier element as a value not representing any feedback operation at a band of a microwave signal.

CONSTITUTION: The polarized plane of a microwave signal incoming to a primary radiator 1 is selected by a selector 2, and further converted into a coaxial mode by a waveguide-coaxial converter 3, amplified by the amplifier element 7 and transmitted from an output terminal 6. The loss of the feedback circuit 8 in a band W of the microwave signal to be amplified is increased and the circuit reaches a state similar to the absence of the feedback circuit, and the incoming microwave signal at a terminal 5 is amplified by the element 7 and transmitted from a terminal 6. On the other hand, the loss of the circuit 8 is small at a band lower than the band W and shows the feedback operation. As a result, the input impedance at the terminal 5 in case of impedance mismatching is brought into a region without any negative resistance and the element 7 attains stable amplification without causing any oscillation.