A dielectric substrate is provided between a first waveguide member and a second waveguide member. The second waveguide member includes a plurality of slots for radiating a horizontally polarized wave. The dielectric substrate is provided with a plurality of line-shaped conductors arranged obliquely with respect to the waveguide axial direction and waveguide wall conductors as waveguide wall surfaces, and includes conductor-removed portions that function as vertically-polarized-wave radiation slot parallel to the waveguide axial direction. The waveguide wall conductors are connected by vias.