PURPOSE: To make the resistivity of a conductive coat proper and prevent abnormal discharge from occurring by coating to sinter oxide powder and fluorescent substances on the conductive coat and replacing to heat and discharge the inside of a bulb with inert gas, and heating to discharge it.

CONSTITUTION: A conductive coat is applied to the inside of a bulb. By coating powder oxide such as γ alumina and fluorescent material on this coat, the fluorescent coat is formed. Then the inside of a bulb is replaced with inert gas, heated, and discharged. As the inert gas, besides nitrogen and argon, neon is desirable. That is, the gas which properly reduces the conductive coat and decreases its resistivity will do.