PURPOSE: To carry out condensing of gas refrigerant in an intermediate heat exchanger smoothly to make the intermediate heat exchanger function as a liquid storage for un-evaporated liquid refrigerant from an evaporator by a method wherein an outlet of the evaporator is connected to a lower part of a falling part, a liquid phase pipe of a liquid-gas separator to an upper part of a rising part through a pressure reducer, and a suction side pipe of a compressor to the lower part of the rising part.

CONSTITUTION: Un-evaporated refrigerant having a relatively low boiling point flows from an evaporator 14 to a falling part 6b of an intermediate heat exchanger 6, evaporates there, ascends while evaporating, enters a rising part 6a from above and descends. At this time, the liquid regrigerant having a relatively high boiling point joins together from a liquid phase pipe 9, and the liquid refrigerant evaporates and returns from the lower part of the rising part 6a to a compressor 3. The gas refrigerant ascending along a gas phase pipe 8 is pre-cooled in the rising part 6a, and descends while condensing satisfactorily in the falling part 6b by evaporation of the refrigerant having low boiling point. That is, the intermediate heat exchanger 6 with a simple construction functions as a storage for the unevaporated refrigerant from the evaporator 14 and a condenser for the gas refrigerant having a relatively low boiling point.