To provide an air conditioning system that properly cools an engine bleed air in an easy structure.

An air conditioning system includes: an air cycle machine ACM that includes first and second turbines 4 and 5 and a compressors 3; a bootstrap circuit 6 communicating with the outlet of the compressor 3 and the entrance of a first turbine 4; a low temperature air passage 7 disposed to the upstream of the compressor 3 and communicating with a first heat exchanger 12, a second heat exchanger 14 that is disposed in the bootstrap circuit 6, the outlet of a first turbine 4 and the entrance of a second turbine 5; and an air supply passage 9 communicating with the outlet of the second turbine 5 and a pressurize chamber 23. The air conditioning system further includes a circulation passage 17 that makes a portion of the air that circulates in the low temperature air passage 7 circulate and return in the low temperature air passage 7; a regenerative heat exchanger 21 that performs heat exchange between the air in the circulation passage 17 and the air in the bootstrap circuit 6; and a flow rate control device 26 that controls a flow rate of the air that circulates in this regenerative heat exchanger 21.