A process-gas supply system 2 supplies a process gas diluted with a diluent gas to a gas using system 4. The process-gas supply system 2 includes a process gas tank 10, a diluent gas tank 12, a main gas duct 14 connecting the process gas tank 10 and the gas using system 4, and a diluent gas duct connecting the diluent gas tank 12 to the main gas duct. The respective main gas duct 14 and the diluent gas duct are provided with flow rate controllers FC1, FC2, and FC5. The diluent gas duct is connected to the main gas duct at a position on an immediately downstream side of one of a plurality of flow rate controllers other than the flow rate controller on the most downstream side. There is further provided a surplus-gas discharge duct 24 through which a surplus diluted process gas is discharged, the surplus-gas discharge duct 24 being connected to the main gas duct at a position on an immediately upstream side of one the flow rate controllers other than the flow rate controller on the most upstream side.