To reduce installation space.

A transformer tank 1 is divided into three chambers 12 to 14, and the first divided chamber 12 is provided with a space, including the bottom of the transformer tank to the inner wall surface of a transformer cover 2, as well as the contents 3 of the transformer on the sidewall inside of the transformer tank, and the space is sealed with insulation oil. A second divided chamber 13 is filled with the insulation oil at an appropriate depth in its inside, and the space over the oil level is sealed with nitrogen gas. The third divided chamber 14 houses a tap switch 4 during loading, and a tap wiring from the tap switch is connected with the contents of the transformer in the first divided chamber, and the insulation oil is injected into the inside. The inside of the first divided chamber is communicated with that of the second divided chamber, by means of a connection tube 8, that enables the insulation oil to be reciprocated between the first and second divided chambers through siphon effect. In addition, a radiator 9 is fitted by a prescribed length in the vertical direction, on the other surface of the sidewall of the transformer tank which constitutes the first divided chamber.