Abstract: A waveguide device for use in propagating an electromagnetic wave of a band having a shortest wavelength ?m in free space includes: a first electrically conductive member having an electrically conductive surface and a first throughhole; a second electrically conductive member including a plurality of electrically conductive rods each having a leading end opposing the electrically conductive surface, the second electrically conductive member having a second throughhole which overlaps the first throughhole as viewed along an axial direction of the first throughhole; and an electrically-conductive waveguiding wall at least partially surrounding a space between the first throughhole and the second throughhole and being surrounded by the plurality of electrically conductive rods, the waveguiding wall allowing the electromagnetic wave to propagate between the first throughhole and the second throughhole. The height of the waveguiding wall is less than ?m/2.