To automatize a welding process by eliminating cracks in an insulating coating (and a black coating) formed on an end of a heater when welded to a heater support.

A heating core is coiled around a mandrel into a helical structure and then the mandrel is dissolved and removed. The heating core forms the heater 25 built in an indirectly-heated cathode structure constituting an electron beam generating part of an electron gun. The heater support 33 for holding the heating core with its respective end parts welded thereto comprises components made by pressing a high-melting point metallic material undissolved by a dissolving liquid for dissolving/removing the mandrel in the manufacturing process of the heating core.