To provide a gas separation system reducing the required membrane area of a gas separation membrane compared with a conventional one and capable of obtaining low permeable gas of high purity at a high recovery rate in regard to a single gas separation system to which two-stage gas separation membrane units are connected in series.

The gas separation system for recovering unpermeated gas from mixed gas using the gas separation membrane includes a line to which a first gas separation membrane unit and a second gas separation membrane unit are connected in series to supply the second gas separation membrane unit with the unpermeated gas exhausted from the first gas separation unit, and a line for supplying the first gas separation membrane unit with permeated gas exhausted from the second gas separation membrane unit, wherein the gas separation selectivity of the first gas separation membrane unit is higher than that of the second gas separation membrane unit.