Abstract: An air-cycle air conditioning system comprises a first turbine disposed in an exhaust passage of a prime mover and driven by the energy of exhaust gases, a first compressor placed in coaxial relation to the first turbine and rotatable in response to the operation of the first turbine so as to intake and compress an air, a second turbine coupled to an outlet end of the first compressor through a first line and rotatable by the energy of a compressed air from the first compressor so as to expand and cool the compressed air, a first heat exchanger disposed in the first line and effecting heat exchange between the compressed air and an outside air, a second heat exchanger connected to the second turbine through a second line and effecting heat exchange between the air supplied from the second turbine and an air supplied from a fan, a second compressor placed in coaxial relation to the second turbine and rotatable in response to the operation of the second turbine so as to intake and compress an air, an auxiliary