PURPOSE: To easily and highly precisely adjust the composition of a gaseous mixture.

CONSTITUTION: A first gas (air) is supplied to a mixing part 41 through a constant-pressure valve 56, and second gases (gaseous oxygen and carbon dioxide) are supplied to the mixing part 41 through constant-flow control valves 65 and 75 to form a gaseous mixture. The pressure of the gaseous mixture is controlled by the constant-pressure valve 56, the flow rates of the oxygen and carbon dioxide are adjusted by the constant-flow control valve 65, hence the pressure of the gaseous mixture is kept constant (flow rate of the gaseous mixture is also kept constant), the flow rate of the air is automatically adjusted, and the composition of the gaseous mixture is easily controlled with high precision.