PURPOSE: To prevent the freeze of cooling medium by connecting in parallel a pipe having a valve in the middle with a system in which valves are arranged on the inlet and the exilt sides of a cooling apparauts which re-cools cooling medium, and in which only the cooling medium in the cooling apparatus can be discharged.

CONSTITUTION: Valves 8 and 9 are arranged on the cooling-water inlet and outlet sides of a cooling apparatus 2, and a valve 10 is arranged in the middle of a bypass pipe 11. In normal operation, the valve 10 is closed, and the valves 8 and 9 are opened, and cooling water is circulated in the cooler 2. If electrical facilities stop, the outside air temperature lowers, and the freeze of cooling water is alarmed, the valves 8 and 9 are closed to cut-off the cooler 2 from the cooling system, and the valves 12 and 13 are opened to discharge only the cooling water in the cooler 2 from the valve 12. Further, the valve 10 is opened, and a circulating pump 5 is operated to circulate cooling water through the bypass pipe 11, and freezing is prevented. Thus, the freeze of cooling medium can be prevented