H plane and E plane of a second rectangular waveguide element are inclined at an angle of 45.degree. with respect to H plane and E plane of a first rectangular waveguide element. A connection element disposed between the first and second rectangular waveguide elements has an inner periphery that surrounds a central axis extending in a direction of electromagnetic-wave propagation. The inner periphery includes surfaces parallel to H plane and E plane of the first rectangular propagation path element, and these surfaces form a staircase such that abutting sections between the surfaces parallel to H plane and the surfaces parallel to E plane constitute projections. The staircase is inclined in a direction corresponding to a direction in which H plane of the second rectangular propagation path element is inclined. Accordingly, an electric field is concentrated in the projections of the connection element, and a plane of polarization of an electromagnetic wave propagating through the connection element is rotated from a plane of polarization in the first rectangular waveguide element towards a plane of polarization in the second rectangular waveguide element.