When a process gas obtained by mixing a source gas with a diluting gas is supplied to a predetermined process apparatus, by supplying the process gas to the predetermined process apparatus through a gas contacting part connecting at least one source gas supplying pipe with at least one diluting gas supplying pipe, no chance that the source gas is exposed to air contamination is provided. Moreover, in particular, by forming the gas contacting part which is to be in contact with the above source gas, diluting gas and the like using such a material as metal, ceramic or the like, emission of materials having bad influences on the process gas, such as organic materials in particular, is eliminated. Furthermore, by providing a branching pipe or exhausting pipe, the degree of diluting the source gas can be varied stepwise, and by further combining with a flow rate adjuster, it can be varied continuously.