To reduce air resistance during flight while preventing occurrence of a surging phenomenon of a fan for driving a drive shaft of an air cycle machine by securing cooling performance in flying in high altitude.

In an air-conditioning system 1 arranged in an airplane mounted with multiple electronic apparatuses, and including: a fan F located in an external air passage 11 for driving an air cycle machine ACM formed by connecting turbines 4, 5 driven by adiabatically-expanding engine extraction in the inside, and a compressor 3 rotating with the turbines to compress the engine extraction to be discharged to the turbines; a primary heat exchanger 12 for cooing the engine extraction by heat exchange with the external air; and a secondary heat exchanger 14 for cooling the air compressed by the compressor and raised in temperature with heat exchange with the external air, a bypass passage 20 for circulating the external air therethrough without passing through the primary heat exchanger 12 and the secondary heat exchanger 14 is formed in the external air passage 11.