PURPOSE: To alleviate a stress at the corners of tank when an internal pressure increases during internal flashover fault by separating the welding line of side plates which form a tank from the tank corners, attaching a reinforcing beams and welding side plates through a plate.

CONSTITUTION: A cover 1 housing the transformer body and insulating oil and a tank 2 are hermetically assembled by welding by sealing the flanges 3A, 3B attached respectively to the apertures. The tank 2 is formed so that the side plate is bend at the corner and the seaming part 12 for welding of side plate is separated from the tank corners X. The reinforcing beam 5 is welded to the side plate forming the tank 2 through the attaching plate 11 at the tank corners X.