NOVELTY - A fluorine gas production unit comprises an electrolytic cell with a molten salt containing hydrogen fluoride for generating fluorine gas as product gas in first gas phase on anode side and hydrogen gas as by-product gas in second gas phase on cathode side; two pipes (52, 72) for deriving the product gas and by-product gas; a deriving flow rate control section (84) in the second pipe and two dilution pipes (82, 83) for introducing inert gas.    USE - For producing fluorine gas in an automated production system e.g. semiconductor device manufacturing system.    ADVANTAGE - By lowering the hydrogen concentration in gas in the second gas phase portion on the cathode side, the pressure generated through the mixed contacting reaction of the gas on the cathode side is lowered. Since load on the electrolytic cell is lightened, damage in an abnormal situation can be restrained in minimum. By equalizing the pressures in two gas phase portions, variation in the liquid level in the electrolytic cell is nearly caused to disappear, it is therefore possible for the unit to work with high security and reliability even when operated for longer period of time. Introduction of inert gas to the second gas phase portion on the cathode side, lowers the hydrogen concentration in gas on the cathode side.    DETAILED DESCRIPTION - The first dilution pipe introduces inert gas into the second gas phase portion so that the hydrogen concentration in the second gas phase portion gets 70 - 10% and the second dilution pipe for introducing inert gas into the second pipe so that hydrogen concentration in gas coming out of the second pipe gets less than 4%. The unit further comprises a pressure control mechanism for equalizing the pressures in the first and second gas phase portions to each other. The driving flow rate control section is used as part of the pressure control mechanism.    DESCRIPTION OF DRAWING(S) - The figure shows a supply pipe system in fluorine gas production unit.    cathode room (36)    first pipe (52)    second pipe (72)    control valve (74)    vacuum generator (75)    inert gas source (80)    first dilution pipe (82)    second dilution pipe (83)    flow rate control section. (84)