An invention comprises: a circular waveguide that has a cylindrical section having a circular pipe conduit with a circular shaped cross section, and side wall sections joined to the both sides in an axial direction of the cylindrical section; a first rectangular waveguide that has a first rectangular pipe conduit with a rectangular shaped cross section and that is joined to one of the side wall sections so that the first rectangular pipe conduit communicates with the circular pipe conduit; a second rectangular waveguide that has a second rectangular pipe conduit with a rectangular shaped cross section and that is joined to the other of the side wall sections so that the second rectangular pipe conduit communicates with the circular pipe conduit; and a dielectric plate that is configured as a plate shape, is disposed in the circular pipe conduit, and is airtightly held to the cylindrical section, wherein the circular waveguide has a plastically deformable section that is plastically deformable so that at least the length in an axial direction of the circular waveguide can be changed.