In a gas separation system, a retentate gas discharge port of a first unit U1 and a gas inlet port of a second unit U2 are connected by a retentate gas discharge line. A permeate gas discharge port of U1 and a gas inlet port of a third unit U3 are connected by a permeate gas discharge line. A feed gas mixture supply line is connected to a gas inlet port of U1. A permeate gas discharge port of U2 and the feed gas mixture supply line are connected by a permeate gas return line. A retentate gas discharge port of U3 and the feed gas mixture supply line are connected by a retentate gas return line. At least in operation, the gas permeability of U2 is higher than that of U3, and the gas selectivity of U3 is higher than that of U2.