PURPOSE: To prevent thermal stress breakdown due to difference in expansion coefficient by diffusing carbon in phases from the surface of a cathode filament to phase dot carbon concentration from the surface to the inside.

CONSTITUTION: A cathode filament 7 is heated in such an atmosphere that organic material containing carbon such as benzene, methane and butane exists to form a relatively-thin primary carbon layer. Next, it is heated in a non- carbide atmosphere and carbon is diffused to form a secondary carbon layer. With the repetition of such treatment, tertiary and quaternary carbon layers are formed. As a result, the carbon layers are diffused inside thereof in repetition so that the carbon layer 7B has carbon concentration phased down from the surface to the inside to provide thermal stress prevention. It is thus possible to manufacture a reliable magnetron, etc., at a low cost.