A waveguide connecting structure includes an inserting waveguide having an inserting conduit line and a flange extending outwardly in a conduit radial-direction, and a receiving waveguide having a receiving conduit line, a receiving structure into which the inserting waveguide is inserted, and stub grooves disposed on both sides of the receiving conduit line outwardly in the direction. The receiving structure has a receiving end face extending in the radial direction and opposing to a flange end face, and an annular receiving inner-circumferential surface disposed outward of the flange and extending in a conduit axial-direction. An electric length of each stub groove in the axial direction from an opening first end to a closing second end is ½ of a conduit wavelength of the stub groove.