PURPOSE: To prvent the penetration of confined metal into the wall of a tubular bulb and keep electric characteristics and luminous characteristics for a long time by a simplified method in which a specific thickness of silicon nitride film is formed on the inner surface of the tubular bulb.

CONSTITUTION: The inner surface of a tubular bulb 4 of a luminous tube 3 for a metal vapor discharge lamp is provided with a thickness of 30W3,000 of a silicon nitride film by a procedure in which a quartz tube is heated in a heating furnace of a hydrogen gas atmosphere while flowing a mixture of silicone tetrachloride and ammonia, etc., in the quartz tube, or the tubular bulb of the luminous tube 3, to crystallize silicon nitride on the inner surface of the quartz tube by reaction. Thus, the electric and luminous characteristics of the luminous tube can be kept for a long length of time without penetration of the confined metal vapor into the wall of the tubular bulb and longer working life and a stable and great luminous intensity can be obtained.