PURPOSE: To prevent the communication between a condenser and an air phase section of a vaporizer by way of an overflow pipe by connecting an overflow pipe of refrigerant between a condenser or a refrigerant flow down pipe and a vaporizer and installing a refrigerant flow passage which replenishes the refrigerant to this overflow pipe.

CONSTITUTION: When the temperature of cooling water is low and the differential pressure between a condenser 3 and a vaporizer 4 is excessively small compared with the discharge pressure of a refrigerant pump 10, or when the discharge pressure of the refrigerant pump 10 is excessively large compared with the above pressure, the liquid level in a refrigerant flow down pipe rises and further raises a refrigerant sump 3A. When the liquid level reaches an opening of an overflow pipe 38, the refrigerant solution flows a refrigerant sump 4A of the vaporizer 4 by way of the overflow pipe 38, which makes it possible to prevent the generation of cavitation of the refrigerant pump 10 or the reverse flow unvaporized refrigerant of the vaporizer to the condenser 3.