The flow rate measuring method is performed in a common gas supply system comprising a plurality of gas supply paths each having a first valve, and a gas measuring device formed downstream side of the plurality of gas supply paths, having a pressure sensor, a temperature sensor, and a downstream side second valve. The flow rate measuring method includes: a first step of opening any one of the first valves and the second valve to allow gas to flow, closing the second valve while gas is flowing, and closing the first valve after a predetermined time has elapsed, and then measuring a pressure and a temperature after the first valve has been closed; a second step of opening any one of first valves and the second valve to allow gas to flow, closing the any one of the first valve and the second valve at the same time while gas is flowing, and then measuring a pressure and temperature after the first valve and the second valve have been closed; and a third step of calculating the flow rate in accordance with the pressure and temperature measured in the first step and the pressure and temperature measured in the second step.