To provide an air conditioner for aircraft which obtains a required cooling function by reducing the bleed air rate from an engine as much as possible and takes the open air as a part of the bleed air.

When grounding or flying at a low altitude, the engine bleed air is cooled by a primary heat exchanger 3a, and on the other hand, the open air 16 is compressed by a compressor 4a, and cooled by a secondary heat exchanger 5a, and the bleed air and the open air are led into a turbine 10a at the same time, and adiabatically expanded, to lower the temperature of the air, and this low-temperature air is fed to a mixing chamber 11. When flying at a high altitude, the engine bleed air is led to the turbine 10a through the primary heat exchanger 3a, and expanded, while insulating the heat, to lower temperature of the air, and on the other hand, the open air 16 is compressed by the compressor 4a, and passed through a secondary heat exchanger 5a, and a flow rate/surge control valve 17a is opened to feed the compressed open air 16 to an outlet side of the turbine 10a, and fed to the mixing chamber 11 with the low-temperature bleed air, and supplied to a cockpit cabin 12.