According to one embodiment, an introduction-side rectangular waveguide tube and a lead-out-side rectangular waveguide tube are each connected to a circular connection plate connected to an end surface of the circular waveguide tube, and at least one connection plate includes, in an outer circumference thereof, the other welding collar welded to the one welding collar, and both the one welding collar and the other welding collar include grooves or holes for a rotation regulation jig to regulate rotation in a circumferential direction, and an entire portion along the circumferential direction including the grooves or holes is welded and fixed thereto.