PURPOSE: To prevent corrosion and contrive high efficiency of cooling by forming a required gap in the outside of an inner tube in which a high level package is housed to be covered with an outer tube and providing the upper end opening and the lower end opening of the outer tube with an upper water receier and a lower water receiver.

CONSTITUTION: In a high level package 8 housed in an inner tube 1, cooling water of a cooling water tank 10 is supplied to a lower water receiver 3 from a cooling water supply tube 4 by the use of a pump 9 and allowed to flow between the inner tube 1 and an outer tube 2 from the water receiver 3 to cool. Thereafter, the cooling water is discharged to the tank 10 from an upper water receiver 5 through a cooling water drain tube 6. The cooling water delivered from the drain tube 6 is circulated to the supply tube 4 again. Thereby, since the package 8 can be cooled from the outside of the inner tube 1, high cooling efficiency can be obtained by a small quantity of cooling water and corrosion can be prevented because the cooling water does not directly come in contact with the package 8.