PURPOSE: To eliminate a spurious resonance mode by inserting plural conductive rods into the H surface of a coaxial conversion waveguide at every 1/4 wavelength so as to adjust them.

CONSTITUTION: N-number (integer of n≥1) of conductive rods 1 and 2 are inserted from the H-surface of the coaxial conversion waveguide 19 in parallel to an electric field at the intervals of about 1/4 wavelength of a basic resonance mode frequency so that they come to the length of 1/4 wavelength of the spurious resonance mode frequency. Consequently, the conductive rods 1 and 2 are inputted to a line in the form of parallel susceptance elements in the center frequency area of a fitter and n-number of conductors are arranged at the intervals of about 1/4 wavelength of the basic resonance mode frequency. Thus, the conductors change from capacitive to inductive by adjusting the parallel susceptance. Thus, matching is attained because n-number of the conductors can independently be varied. when the length of the rods is changed by parallel reactance, the conductive rods 1 and 2 resonate in series in the high area side of the filter and they operate as serial resonance elements. Thus, the spurious modes of dielectric resonators 6-10 can be suppressed.