PURPOSE: To make flat the frequency and phase characteristics of the mixer output signal, by rejecting the harmonics produced at the mixer through the use of the harmonic blocking filter and harmonic absorbing body.

CONSTITUTION: The open stubs 9 and 10 of 1/4 wave length to the harmonics as the harmonic blocking filters are placed between the output side of the hybrid circuit 3 inputting the signal at the input terminal 1 and the signal of the local oscillator 3 and the mixer diodes 4 and 5, and the band rejection filters 11, 12 are connected at the output side of the diodes 4 and 5 to reject the frequency of the input signal f1 and the local oscillation frequency f2 and to pass through the intermediate frequency and harmonics of the frequency f3. The harmonics which pass through the filters 11 and 12 are absorbed with the microwave absorbing bodies 13 and 14, provided at the output side of the filters 11 and 12 to output the intermediate signal in which the harmonics, input signal and local oscillation signal are rejected at the output terminal 8.