To improve a mounting property of an air conditioner on a small aircraft, to prevent bedewing of a front window glass and to improve a field of view.

A condenser fan 5, a centrifugal fan 6, a condenser 42, an evaporator 44 and a heater core 32 are stored inside an air conditioning duct 4. Additionally, an air passage in the air conditioning duct 4 is partitioned into an outside air passage 19 to be a condensing room and an inside air passage 20 to be a cooling room by first to third inside and outside air change-over dampers 16-18. Consequently, the inside of a cabin slightly heated and dehumidified by reheating inside air absorbed in the air conditioning duct by the heater core 32 after cooling and dehumidifying it by the evaporator 44 while reducing a load of an engine for flight by motive-power-saving-driving a compressor stopping the condenser fan 5 and driving the centrifugal fan 6 and a water pump at the time when a small aircraft flies.