PURPOSE: To prevent the stiffening of a metal coil during the heater operation and extend its life by providing a metal thin film made of a specific metal element between the metal wire coil and an insulating ceramic film.

CONSTITUTION: A metal thin film made of a metal element which is other than the main component element of a metal coil 1, is not sublimated or melted at the temperature up to 1500°C and in the vacuum of 10-7. Torr or less, preferably to 10-8, prevents the inevitable impurities in an insulating alumina film 4 provided on it from being diffused into the metal coil 1, and does not deteriorate the tensile strength of the metal coil 1 even if it is diffused into the metal coil 1, and the insulating alumina film 4 on the metal thin film are attached on the spirally wound metal coil 1 heating the electron beam emitting cathode pallet 3 of an electron tube. The stiffening of the tungsten coil 1 caused by the ion species emitted from a cathode sleeve 2 and the element species evaporated from the alumina insulating film 4 can be prevented.