PURPOSE: To provide an air cycle air conditioning system, which is improved in energy efficiency remarkably rather than the system of conventional techniques so as to be adapted to airplanes especially, and an air conditioning method therefor.

CONSTITUTION: The air cycle air conditioning system is provided with an air-air heat exchanger 17 defining a first air flow path for air to be cooled and a second air flow path for air to pass therethrough and for absorbing heat from the air to be cooled and an apparatus 18 for supplying air to the said second air flow path at sub-atmospheric pressure.