PURPOSE: To prevent breakdown of duct through alleviation of increase of internal pressure of duct at the time of internal flashover fault and also prevent secondary failure due to the injection of oil and gas by forming polygonal duct comprising oil-filled lead having the specified length of each side.

CONSTITUTION: A transformer body 1 is housed in a tank 3 with an insulating oil 2. The tank 3 is provided with a low voltage line extending bushing 4 and a wall-through bushing 5 for extending a high voltage line, and is connected with the transformer body 1 by the leads 8 and 9. Moreover, the high voltage path end is housed in a cable duct 7 partitioned by the bushing 5 with an insulating oil 2 and is extended to the outside after it is connected to a cable head 6 by the lead 10. The cable duct 7 is formed with the N-polygonal cross-section.