Branch piping (18) branches off from the upstream side of opening/closing valves (13d, 14d) provided near the entrance of a processing chamber (11) of a gas supply system for supplying a processing gas, and the branch piping (18) is connected to gas discharge piping (17). In the branch piping (18) are provided a gas flow rate detection mechanism (19) and opening/closing valves (13h, 14h) for switching a flow path between the processing chamber (11) side and the branch piping (18) side. The gas flow rate detection mechanism (19) causes a gas to flow through a resistance body to measure a pressure across the resistance body, detecting a gas flow rate from the pressure difference. Mass flow controllers (13a, 14a) are tested or corrected by the detected value.