A traveling-wave tube, including a delay line having two end surfaces, a spatially periodic permanent magnet system surrounding the delay line for the bunched guidance of an electron beam, the delay line having line cells disposed in tandem, each of the cells having an electron beam passage opening formed therein and adjacent partitions separating the cells with a given mutual spacing, each of the partitions having at least one coupling opening formed therein extended substantially in circumferential direction, a ridgeless rectangular waveguide coupled to at least one of the end surfaces of the delay line and protruding away from the delay line substantially at right angles to the axis of the delay line, the waveguide having a location of smallest reflection, a wide side facing away from the delay line extended substantially perpendicular to the longitudinal axis of the delay line and at least one narrow side being steadily tapered toward the delay line down to substantially the given spacing of the adjacent partitions, and a short-circuit slider disposed below the waveguide, includes a hollow-cylindrical matching element disposed at the wide side of the waveguide facing away from the delay line, the hollow-cylindrical matching element having a fine thread screwed to the location of least reflection and fixed and soldered to the waveguide.