PURPOSE: To prevent the generation of a corona discharge in each position of a tube by returning a heat-exchanged cooling water to the outside through the returning tube of a cooling water circulating tube.

CONSTITUTION: When pure water is supplied to pass cooling pipings 31a, 31b arranged under a ground bottom 2, cooling water is circulated in cooling tubes 11a, 11b spirally wound in a hollow insulator 21 to cool each device in a high voltage terminal 3. The heat-exchanged cooling water is returned to the outside through the returning tube part of the tubes 11a, 11b. A prescribed voltage is applied to the high voltage terminal 3 by a high voltage power source, and the pure water is passed in the cooling water tube 11. A current according to a resistance value determined by the product of the specific resistance of the pure water and the length of the tube 11 is carried from the terminal 3 to the ground bottom 2. Since the tube 11 is exposed to an insulating gas SF6 charged in the hollow insulator 21, the voltage is stably applied without generating a corona discharge in each part.