PURPOSE: To make high frequency characteristics uniform over the entire lifetime of wave-traveling tube elements regardless of a change in voltage applied to an anode electrode by providing an electromagnet for graduating a focused electric field, in addition to a permanent magnet, as a magnet for focusing an electron beam.

CONSTITUTION: An electromagnet 4 is provided for graduating a focused electric field. As this electromagnet 4, a thin type electromagnet called a print coil is used for enabling its insertion in a space between permanent magnets 3. In an initial stage for using a traveling-wave tube, the electromagnet 4 is not energized and only the electric field of the permanent magnets 3 are employed for the graduation of an electron beam. When voltage applied to an anode electrode is changed with the deterioration of a cathode electrode 1 in a latter stage for using the traveling-wave tube, the electromagnet 4 is energized so as not to cause the change of an electron beam focusing condition before and after the voltage change.