PURPOSE: To improve the characteristics of conversion loss, by providing a coupling short-circuited section with a part between coupling lines, in a microwave mixer circuit including parallel coupling lines having an open stub.

CONSTITUTION: To short-circuit the impedance viewed from lines 18, 19 to which mixer diodes 20, 20 are connected, a coupling short-circuited section 16 is provided at a specific location of an inverse phase transformer consisted between joints 33-32 and 35-38 of parallel coupling lines 36, 37 which are determined with the characteristic impedance and the electric length of an open stub 14, so that the sum component of local oscillation signal and reception signal produced in the mixer diodes can completely be reflected at the lines 18, 19.