PURPOSE: To effectively cool a coil by a construction wherein the coil with high calorific value is cooled by evaporation heat of a condensable liquid and other parts are cooled by non-condensable gas.

CONSTITUTION: When coils 3, 4 housed in a coil container 6 generate heat, a condensable liquid 13 is evaporated and vapor 14 in the form of air bubbles ascends to be stored in a vapor reservoir 22. Then, the vapor 14 enters into a heat exchanger 17 from an outlet port 15 through an outlet pipe 18. The heat exchanger 17 includes plural heat exchanging pipes 17a, and the vapor 14 is liquidized while passing through the heat exchanging pipes 17a. Thus liquidized condensable liquid 13 is returned to the container 6 from an inlet port 16 through an inlet pipe 19. Meanwhile, other heating parts (such as an iron core 2) except for the coils are cooled by non-condensable gas 20 filled in a space within a tank 1 other than the coil container 6. The heated non-condensable gas 20 enters into an external cooler 21 from a cooler inlet 23 so as to be cooled, and after being cooled it returns to the tank 1 from a cooler outlet 24.