A waveguide device includes an electrical conductor including an electrically conductive surface, a waveguide extending alongside the electrically conductive surface, and an artificial magnetic conductor extending on both sides of the waveguide. The waveguide includes a first portion extending in one direction, and at least two branches extending from one end of the first portion, the at least two branches including a second portion and a third portion that extend in mutually different directions. A waveguide defined by the electrically conductive surface, the waveguide surface, and the artificial magnetic conductor includes an enlarged gap portion at which a gap between the electrically conductive surface and the waveguide surface is locally enlarged.