PURPOSE: To attain a high cooling effect with a small amt. of water by providing a spiral obstacle respectively between a first electrode and a column in the first electrode and between a second electrode and a metallic tube on the periphery of the second electrode.

CONSTITUTION: The cooling water of a first electrode is introduced from an inlet 4b, allowed to flow in spiral water passage partitioned by a spiral wire 10a inserted into the gap between the inner face of a first electrode 1 and a column 9 and discharged from an outlet 5b. The cooling water of a second electrode is introduced from an inlet 4a, allowed to flow in a spiral water passage partitioned by a spiral wire 10b inserted into the gap between the outside of a second electrode 2 and a metallic tube 3 (water cooling pipe) and discharged from an outlet 5a. Since the cross-sectional area of the water passage is decreased, the flow velocity is increased with respect to the same amt. of water, the heat transfer coefficient is increased, and a cooling effect is enhanced.