To exhibit the capacity of a condenser, and thereby the entire system, to the maximum by controlling the degree of supercooling indicative of the degree of supercooled state at the outlet of the condenser to a predetermined level or below.

The degree of supercooling SC means a temperature drop from a reference level, i.e., a supercooled state starting temperature under a specified pressure at the outlet of a condenser 2. It is known from test that the capacity of the condenser 2 decreases abruptly as the SC exceeds 15°C. SC at the outlet of the condenser 2 is detected by a temperature sensor 30 provided thereat and compared with prestored characteristics thus operating SC at that time. Based on anoperated SC, a main controller 29 performs on/off control of a compressor 6 through a clutch controller 4 and a clutch 5 and opening control of an expansion valve 8 thus controlling SC at the outlet of the condenser 2 to a predetermined level or below.