PURPOSE: To improve heat rediation of electrode and heat absorption by forming a black coat on the electrode surface by vacuum deposition or sputtering so as to reduce a gas emission quantity while improving the adhesion strength.

CONSTITUTION: A hollow cylindrical positive electrode 4 is placed on a holder 14 rotatable by a motor 13 on the bottom of a vacuum tank 12. An electrode 16 is provided through a flange 15 at the top of the vacuum tank 12 while a wire 17, which is a chrome-plated molybdenum wire of the diameter 1.0mm, and a jig 18 supporting this wire are connected thereto. After the vacuum tank 12 is exhausted, oxygen is introduced thereto through a conduit 19 from an oxygen cylinder 20 while regulating a variable flow valve 21. Then, a current is supplied to flow through a wire 17 degassed in a vacuum in advance so as to be conduction-heated. During heating, the holder is turned in order to form a black coat 22 of chromium oxide or a metallic chromium on the inside surface of an anode 4. Thereby, the whole gas emission quantity can be made about 20% of a chromeplated film.