To meet the requirements for air-conditioning under all operating conditions of an aircraft, to operate an air-conditioning unit at reduced fuel consumption, and to reduce the weight of the unit.

In this unit, air exhausted out of a pressurized cabin 2 from the pressure control valve 25 of the pressurized cabin 2 is used as a source of cooling air by connecting a duct 43 or the like to a port of a three-way valve 41 or the like other than an outside air inlet port, the three-way valve 41 intervening in an outside air duct 36 through which outside air is introduced for cooling high-temperature, high-pressure air introduced from a bleed air line 30 as a source of conditioned air for an ACS 3, and also for cooling a high-temperature, high-pressure refrigerant a VCS 4. A part or the whole of cooling outside air is substituted to restrain an increase in drag caused by the introduction of outside air during flight, so as to reduce fuel consumption. In an above-ground high-temperature environment wherein cooling outside air is most required, pressurized cabin exhaust of lower temperature is used as a source for cooling the refrigerant of the condenser 21 of the VCS 4. This allows reduction in flow rate of cooling air, whereby the capacity of a fan 23 is reduced and the unit can be miniaturized and made lightweight.