PURPOSE: To attain a filter with small size and high performance by providing 2nd and 3rd coupling posts projected to a waveguide and coupling with a polarized wave perpendicular to the polarized wave front in the waveguide, connecting output terminals of the 1st and 2nd coupling posts and using other terminal of the 3rd coupling post as an input terminal or an output terminal of a signal.

CONSTITUTION: A polarized wave in a longitudinal direction inputted from an input terminal 21 of a waveguide 20 propagates in a circular waveguide 20 by a distance l2 up to the coupling post 23. A signal extracted by the coupling post 23 once is radiated as a polarized wave in the lateral direction from the coupling post 24 via a coaxial line. The polarized wave in the lateral direction propagates a distance l3 up to the coupling post 25 and an output signal is extracted from the coupling post 25. The undesired frequency component lower than the cutoff frequency of a circular waveguide 20 included in the input signal is attenuated while being propagated from the input terminal to the coupling post 28 in the circular waveguide 20 and further attenuated while being propagated from the coupling post 24 to the coupling post 25. Thus, the waveguide filter acts like a waveguide filter having a length of (l2+l3).