PURPOSE: To prolong the life of a cathode and obtain its high efficiency, by allowing a material of specific composition to adhere onto an electrode and forming the cathode.

CONSTITUTION: Between anodes A1, A2 and cathode parts on cathode base plates C1, C2, electric discharge is caused to generate ultraviolet rays, excite a flurescent material P applied on the inner wall of a cell hole H and emit light. As the cathode base plate C1, C2, a ribbon made of, for instance 42-6 alloy is used, and a material E, in which at least one of CaAl4, SrAl4, BaAl4 is contained and their sum total exceeds 50% in weight ratio, is allowed to partially adhere to said ribbon and a cathode is formed.