PURPOSE: To reduce the flow rate of the O2 gas contained in the mixture gas down to a minute level 1CCM or less with the stable flow rate control.

CONSTITUTION: Each of flow rate control valves 1-4 and 27 has its full scale 100CCM. Then the CF4 gas and the O2 gas are mixed together in a ratio of 99%:1%, and the flow rate of the O2 gas is set at 0.5CCM. This O2 gas is supplied into a chamber 30. Meanwhile the flow rate of the control valve 2 is set at 10CCM. Then the open/close valves 6, 14 and 21 are kept open for only one minute. The O2 gas is supplied into a gas container 23 by 10cc, and the flow rate of the control valve 1 is set at 100CCM. The open/close valves 5, 13 and 21 are kept open for only 9 minutes, and the CF4 gas is supplied into the container 23 by 990cc. Then the flow rate of the control valve 27 is set at 50CCM, and the open/close valves 24, 25 and 28 are opened. Thus the mixture gas is supplied into the chamber 30 from the container 23 in a flow rate 50CCM. Under such conditions, the flow rate of O2 gas is set at 0.5CCM.