A gas separation method in which a rare as a first introduced gas and an impurity gas as a second introduced gas, are introduced into a raw material gas. Each of the flow rates of the first and second introduced gases is controlled based on the flow rates of the rare gas and impurity gas in the discharged gas from a rare gas using facility. A gas separation device includes an introduction pipe for introducing rare gas in a separation gas container into a raw material gas, an introduction pipe for introducing impurity gases in the separation gas container into the raw material gas, a flow meter provided in a supply pipe for supplying a discharged gas of a rare gas using facility, and an arithmetic device electrically connected to each of the flow meter the flow rate controller, and the flow rate controller.