An input signal having an intermediate signal superposed on a DC voltage is applied to a variable attenuation circuit through an amplifier. An output of the variable attenuation circuit is detected by a detecting circuit. A DC voltage from the detecting circuit is compared with a reference voltage from a reference voltage generating circuit by a comparing circuit. Based on a comparison output, the amount of attenuation of the variable attenuation circuit is controlled, and after the output of the variable attenuation circuit is amplified by an amplifier, it is mixed with a local oscillation signal from a local oscillation circuit at a mixer circuit, converted to a microwave signal, and transmitted.