Abstract: Shutoff valves are placed in parts of the process gas supply lines at positions near the processing vessel. A main purge gas supply line branches into branch purge gas supply lines, each of which is provided with an orifice called sonic nozzle. The branch purge gas supply lines are connected to parts of the process gas supply lines extending between the shutoff valves and the processing vessel. The ratio P1/P2, where P1 is primary pressure on the primary side of the orifice and P2 is secondary pressure on the secondary side of the orifice, is controlled to be not less than a predetermined value, for example, two, thereby, the purge gas can always be supplied at equal flow rates into the process gas supply lines. The total flow rate of the purge gas is controlled by a mass flow controller placed in the main purge gas supply line.