To supply a gas coping with every testing condition, by adjusting the gas to a stable humidity and adjusting the flow rate to a constant state and enlarging a conditional range of dilution and mixed gas formation before discharging gas of unfavorable conditions with an unstable and narrow conditional range injected to a union of a code 20 by a union of a code 21, converting to a stable gas.

The pressure of the gas is kept constant by controllers of codes 5, 6; the gas is injected into water storing cylinders of codes 1, 2, or into drying cylinders of codes 7, 8; the humidity is adjusted; and the gas is mixed by selector valves of codes 15, 16 to form the diluted gas, which is adjusted by a flow meter of a manufacturing code 19 and discharged.