To provide an air conditioning system for an aircraft, for preventing a trouble such that large drag is generated in an airframe of an aircraft as a large amount of ram air is supplied to a primary heat exchanger and a secondary heat exchanger.

The air conditioning system includes: a ram air door 81 which is provided in a ram air path 8 and is a ram air supply amount change means for changing the supply amount of ram air; a compressor exit temperature sensor 91 capable of measuring the temperature of exhaust at the exit of a compressor 4; a total temperature sensor 92 which is a temperature sensor capable of measuring the total temperature; and an altimeter 93 capable of measuring an altitude further. A target value of a compressor exit temperature is set as a function, of the total temperature indicated by the total temperature sensor 92 and the altitude indicated by the altimeter 93, and the ram air door 81 is controlled so as to bring the temperature of the exhaust indicated by the compressor exit temperature sensor close to the target value.