PURPOSE: To eliminate the deterioration of an error rate when the reception side demodlates the modulation wave by varying the amplitude and phase of an output wave and cancelling the leaked local oscillation component while applying the output of a local oscillator in the same and opposite phase.

CONSTITUTION: The multi-value QAM modulation wave of an intermediate frequency band fed from a terminal IN is fed to a mixer 1. Since an output from a local oscillator 2 is fed to the mixer via a directional coupler 4, they are mixed and converted into the multi-value QAM modulation wave of a microwave band and the leaked local oscillation component is fed to a hybrid circuit 7 from the mixer 1. On the other hand, the amplitude and phase of a part of the output of a local oscillator 2 extracted by the directional coupler 4 are cancelled by the said leaked local oscillation component by varying a variable attenuator 5 and a variable phase shifter 6 and the result is fed to the hybrid circuit 7, then the leaked local component is made completely 0. That is, since a circuit giving effect on the frequency characteristic of the circuit is not inserted, no effect is given on the spectrum of the multi-value QAM modulation wave.