To provide an air-conditioning system for an aircraft whereby outside air is effectively sucked to a ram air circuit, and an extraction pressure is enhanced.

In this air-conditioning system for the aircraft, the air extracted from an engine 1 through a valve 1b is expanded by a turbine 3, an air flow rate at a high speed is sent to the ram air circuit, and the outside air is sucked. The high temperature and high pressure air is taken in from the engine 1 through a valve 1a and is compressed in a compressor 2. After that, the air is heat-exchanged in a primary heat exchanger 4, is compressed by a second stage compressor 5, and is introduced into a high temperature side circuit of a reheater 7 through a secondary heat exchanger 6 to be cooled. Then, the air is entered into a condenser 8 and the moisture is condensed, the moisture is separated in a water extractor 9, and is entered into a turbine 10 through the reheater 7. The air is adiabatically expanded in the turbine 10, is cooled and is entered into the condenser 8, and the low temperature air is sent to the cabin.