PURPOSE: To make it possible to keep excellent concentricity of a tublar enclosure with a periodical permanent magnet by inserting a thin metal plate into a united part composed of the tublar enclosure forming a vacuum vessel surrounding a retardation circuit and periodical permanent magnet.

CONSTITUTION: The microwaves amplifying portion of a helical travelling-wave tube is constitutd by a tublar enclosure 3 forming vacuum vessel surrounding a helix 1 and a helix supporting rod 2. Moreover, a magnet 4 and a pole piece 5 constituting a periodical permanent magnet around the enclosue are provided. A non-magnetic thin metal plate 6 is inserted into a gap between the enclosure 3 and the pole piece 5 depending on the size of said gap in order to provide the good concentricity of the helix with the pole piece 5. Hereby, it is possible to keep excellent electron beam transmission coefficient of a travelling-wave tube.