PURPOSE: To maintain high frequency action characteristic and lifetime characteristic favorably, by keeping a base plate in a higher temperature than its action average temperature, keeping the temperature of the cathode of an electron gun portion in a lower temperature than its action average temperature, and simultaneously adjusting a focusing magnetic field most suitably.
CONSTITUTION: The electron beam focusing of a periodic magnetic field device 13 is carried out, in a condition that respective parts are assembled but a cover 16 is not mounted yet. In this case, a base plate 15 is kept in the almost highest temperature in a temperature in which a communication device using a travelling-wave tube is expected to encounter, for example, by a heater built-in device. On the other hand, the temperature of a cathode in an electron gun portion is set in a lower temperature than its average action temperature. The focusing condition of electron beams by the magnetic field device is adjusted most favorably, in a condition that the temperature of respective parts is stabled. Namely, a periodic magnetic field is adjusted minutely in such a manner that the quantity of the electron beams picked by a slow-wave circuit portion becomes lowest, and the magnetic field device is fixed in the most favorable condition.