A waveguide tube connecting member includes a first waveguide tube having a first waveguide path and a flange. The flange has a flange end surface extending from a first opening end of the first waveguide path toward an outer side in a tube radial direction, and a second flange outer peripheral surface which is a part of a first flange outer peripheral surface. The second flange outer peripheral surface is a surface formed in a shape in which a part of the flange has a cavity. An electric length from the first opening end of the flange end surface to the second flange outer peripheral surface along the tube radial direction is (2×N+1)/4 times a wavelength.