PURPOSE: To produce the band-stop filter only with one mold by providing the groove of a resonantor on one tube wall of a wide width face inside a rectangular waveguide, providing a post for adjustment protruded vertically to the wide width face and providing the post on the same tube wall as the groove while shortening the length of the groove rather than the lateral width inside the waveguide.

CONSTITUTION: A main body 11 is a one-side opened and U-shaped waveguide provided with the dimensions of lateral width (a) and depth (b) and constitutes the rectangular waveguide while being joined with one cover 12. On the wide width face of the main body 11, grooves 13 provided with width W, depth (d) and lateral length (s) are provided at the intervals of 3/4λg (λg is an in-tube wavelength), the depth (d) of each groove is selected to the about 1/2 length of the in-tube wavelength λg at the frequency band to be passed, and the grooves 13 are operated as resonators. On the other hand, a post 14 is installed on the lateral wide width face of the grooves 13. In this case, when constituting the band-stop filter by molding, it is enough only to produce one mold for forming the grooves 13 and the post 14 at the main body 11, and the cover can be one metallic plate.