A waveguide transmitting an electromagnetic wave having an electric field that oscillates in a first direction in a second direction perpendicular to the first direction. The waveguide includes rectangular waveguide portions, and a protruding wall and a retracted wall that connect a rectangular waveguide portion to another rectangular waveguide portion. Each of the rectangular waveguide portions has a tubular shape extending in the second direction, and an inner wall of each rectangular waveguide portion has a rectangular cross section. The rectangular waveguide portions are arranged in the second direction, and inner spaces of the rectangular waveguide portions are connected to each other. The protruding wall extends from one of a pair of side surfaces of the rectangular waveguide portion opposed in a third direction toward the other of the pair of side surfaces, the third direction being perpendicular to the first and second directions.