PURPOSE: To suppress the leakage of a local oscillation signal uniformly over the entire local oscillation frequencies by selecting the length of one microstrip line element of a coupling degree adjusting means having an open stub and adjusting the degree of coupling between both resonance line elements to λ/4 of a desired trap frequency.

CONSTITUTION: Open stubs 56, 58 made of microstrip line elements from a reference potential end at both sides of each resonance circuit are inserted between resonance line elements 34 and 44. The other end of the open stubs 56, 58 is opened and the degree of coupling of the resonance line elements 34, 44 is changed depending on the distance of the open ends. The line length of the microstrip line 58 inserted between both the resonance circuits is selected to λ/4 of the wavelength near the upper limit of the local oscillation frequency for the purpose of the adjustment of the degree of coupling of the 1st and 2nd resonance circuits so as to form a trap near said frequency and increase the attenuation, then the leakage of the local oscillation signal to the input terminal of the 2nd converter is improved.