PURPOSE: To increase a tolerance with respect to a high pressure upon an overload or the like during heating operation and improve operating rate by a method wherein the title air-conditioning machine is provided with a pressure switch, opening first and second opening and closing valves when the detected value of a pressure at the refrigerant discharging side of a compressor has exceeded a set value.

CONSTITUTION: When the refrigerant discharging pressure of a compressor 2 has exceeded the set value of a pressure switch 18 due to the overload or the like of the compressor during heating operation, a pressure switch 18, detecting the exceeded pressure, closes the normal open contacts thereof to excite first and second bypass valves 13, 25 and open them simultaneously substantially. According to these operations, one part of liquid refrigerant, condensed and liquefied in an indoor side air heat exchanger 4 and whose pressure is reduced by a first capillary tube 5a, is guided to a first bypass tube 10, then, the pressure of the refrigerant is reduced further by a third capillary tube 12 and, thereafter, the refrigerant is returned to an accumulator 8. Simultaneously, one part of gaseous refrigerant from a four-way valve 3 is guided to a second bypass tube 11 and the pressure of the refrigerant is reduced by a fourth capillary tube 14 and, thereafter, the refrigerant is returned to the accumulator 8.