PURPOSE: To realize replacement or periodical check of transformer within a short period of time and without power failure by accommodating a transformer within the transformer box provided separately from the transformation tank and by fixing such transformer box in the outside of the transformation tank.

CONSTITUTION: The transformer box 17 accommodating the transformer 5 is formed separately from the transformation tank 21 and is provided at the lower part outside the bushing pocket 18 of the transformation tank 21, causing the insulation oil 14 within the transformation tank 21 and transformer box 17 not to be connected. The transformer box 17 wherein the transformer 5 is placed within the insulation oil 14 is fitted to the bushing pocket 18 with the bolts. At the primary side of the transformer 5, the male attachment terminal 15 supported by the insulation plate 20 is electrically connected through engagement to the female attachment terminal 16 supported by the insulation plate 19 in the side of transformer. The transformer 5 can be removed from the line by releasing fitting of the transformer box 17 to the bushing pocket 18 and simultaneously removing the male attachment terminal 15 from the female attachment terminal 16.