PURPOSE: To perform a suitable room cooling irrespective of atmospheric temperature and a pressure loss of an indoor unit by providing a temperature sensor for measuring a refrigerant temperature in an outlet/inlet of an indoor heat exchanger, and so controlling as to increase or decrease a rotating speed of an engine based on the output of the sensor.

CONSTITUTION: In a gas heat pump air conditioner having a heat pump circuit in which a compressor 2, a four-way valve 3, an outdoor heat exchanger 4, a receiver tank 5, an expansion valve at the time of room heating, an indoor heat exchanger 7 and an accumulator 8 are sequentially coupled, and the compressor 2 is driven by an engine 1, temperature sensors are respectively provided in the refrigerant inlet and outlet of the exchanger 7, and output signals of the sensors are input to a controller 9. A target temperature range for so controlling a temperature of the refrigerant is previously set in the controller 9 that a temperature of blow-off chilled gas falls within a predetermined temperature range. Then, the rotating speed of the engine is so controlled to be increased or decreased based on the real refrigerant temperature as to fall within the target temperature range.