A gas separation system includes an adsorption apparatus and a separation membrane apparatus. A mixed gas including a target gas is introduced into the adsorption apparatus. A replacement gas is further introduced into the adsorption apparatus, and at least part of the gas exhausted from the adsorption apparatus at this time is introduced into a space on the non-permeate side of the separation membrane apparatus to obtain a first enriched gas enriched with the target gas by the separation membrane. Meanwhile, after the introduction of the replacement gas, the gas is exhausted from the adsorption apparatus while causing an adsorbent to desorb or dissipate the target gas so as to obtain a second enriched gas enriched with the target gas.