The purity of a refrigerant liquid can be maintained to an appropriate level without being varied corresponding to a change in the atmospheric condition and the loading state. A portion of the refrigerant liquid in an evaporator is conveyed from the evaporator to the upper end of a rectifier where it is used as a vapor/liquid contact fluid. A temperature sensor T6 is provided at the upper end of the rectifier for detecting the temperature at the top of the rectifier or the temperature of the refrigerant vapor. The temperature and the purity of the refrigerant vapor are closely related to each other. The higher the temperature, the lower the purity is deteriorated. When the temperature T detected by the temperature sensor T6 is higher than a reference temperature level Tref, a flow control valve V5 is actuated to increase the flow of the refrigerant liquid to the rectifier. This recovers the purity of the refrigerant vapor, thus preventing declination of the purity of the refrigerant vapor in the evaporator.