A waveguide device includes: a first conductive member having an electrically conductive surface; a second conductive member having a plurality of electrically conductive rods arrayed thereon, each conductive rod having a leading end opposing the conductive surface; and a waveguide member having an electrically conductive waveguide face opposing the conductive surface, the waveguide member being disposed among the conductive rods and extending along the conductive surface. The waveguide member includes at least one of a bend and a branching portion. A measure of an outer shape of a cross section of at least one of the plurality of conductive rods that is adjacent to the bend or the branching portion, taken perpendicular to an axial direction of the at least one conductive rod, monotonically decreases from a root that is in contact with the second conductive member toward a leading end.