The present invention includes a circular waveguide with a cylindrical shape, a lead-in side rectangular waveguide, and a lead-out side rectangular waveguide. The circular waveguide includes a dielectric airtight window that partitions an internal space into a lead-in side and a lead-out side and one welding collar disposed on the circumference of an end face of at least one of the lead-in side and the lead-out side. The lead-in side rectangular waveguide and the lead-out side rectangular waveguide are connected to circular connecting plates that are connected to the corresponding end faces of the circular waveguide; and another welding collar to be welded to the one welding collar is provided on the circumference of at least one of the connecting plates. A groove or a hole, into which a rotation restriction jig for restricting rotation in the circumferential direction is fit, is formed in each of the one welding collar and the other welding collar; and the entire circumference of each of the one welding collar and the other welding collar including the groove or the hole is fixed by welding.