The present invention provides a gas separation system suitable for reducing energy required to separate a gas mixture. A gas separation system 100 of the present invention includes: a first separation membrane unit 10 that separates a gas mixture 70 containing carbon dioxide and nitrogen into a first permeated gas 80 and a first non-permeated gas 81; a second separation membrane unit 20 that separates the first permeated gas 80 into a second permeated gas 90 and a second non-permeated gas 91;a first decompression device 50 that decompresses a permeation-side space of the first separation membrane unit 10; and a second decompression device 52 that decompresses a permeation-side space of the second separation membrane unit 20.