A dielectric resonator is housed within an internal space of a metallic casing. An amplifier is disposed outside the space and an input and output circuits are connected to the amplifier. An output coupling window is formed in the casing at the position corresponding to the output circuit of the amplifier and an input coupling window is formed in the casing at the position corresponding to the input circuit of the amplifier. An electromagnetic wave leaking from the output circuit of the amplifier is coupled through the output coupling window to the dielectric resonator and the dielectric resonator is further coupled to the input circuit of the amplifier through the input coupling window.
