PURPOSE: To obtain a larger amount of heat transfer by providing a communicating hole of a check valve inside a gas-liquid separator to allow liquid medium in a receiver to flow into the gas-liquid separator within a short time period.

CONSTITUTION: A communicating hole of a check valve 7 is provided inside a gas-liquid separator 3. Thereby, as the differential liquid level between a receiver 2 and the check valve 7 becomes large, the time period required for liquid medium in the receiver 2 to flow completely into the gas-liquid separator 3 can be shortened, so that a large amount of heat transfer can be obtained. Furthermore, as the liquid medium may flow into the gas-liquid separator 3 via the shortest route, the vertical arrangement of the receiver 2 and the gas- liquid separator 3 can be made small and compact. As the lower end of a double valve case 5 connected to the gas-liquid separator 3 is located above the communicating hole, resistance across the double valve case 5 becomes nil completely when the liquid medium flows out so that the time can be shortened further and a large amount of heat transfer can be obtained.