PURPOSE: To prevent overcooling of a room and prevent refrigerant, returning to a compressor, from being left in liquid state as it is by a method wherein the circulating amount of the refrigerant is controlled so that the downstream side of an indoor heat exchanger will never become superheated condition upon low-humidity dry operation.

CONSTITUTION: When low-humidity dry operation is started and the temperature of an indoor heat exchanger 5 is stabilized, the temperature of an upstream side hair pin for the indoor heat exchanger is compared with the temperature of a downstream side hair pin for the same by temperature sensors 6, 7. When the upstream side temperature is higher than the downstream side temperature, an expansion valve 4 is choked by two stages to stabilize the temperature of the indoor heat exchanger 5. The upstream side temperature and the downstream side temperature are detected again to judge whether the upstream side temperature is higher than the downstream side temperature by 5°C or higher or whether the downstream side of the indoor heat exchanger 5 is superheated or not and when it is so, the expansion valve 4 is loosened by one stage to obtain the temperature difference of 3-5°C. According to this method, overcooling of a room can be prevented.