PURPOSE: To allow improvement in cooling performance and control of temperature rise, especially at bushings by providing vents on a bottom surface and an upper part of a terminal box covering the upper part of a heat dispersion part and equipping a blast means for circulation of cooled air flow.

CONSTITUTION: A terminal box 4 for concealing bushings 3 is provided over an electrical device itself and a tank 1 containing insulating oil therein and having a heat dissipating part 2 such as a wave-shaped rib on the side thereof, and cables 6 are drawn out of lower parts of the both sides. A slit-like vent 8 is provided on the lower surface of the terminal box 4 and a hand hall is provided over the terminal box 4. A propeller fan 10 is mounted on a cover 5 of the hand hall and a vent 9 having a drip-proof cover is provided over the cover. Thus, external air is drawn up from the vent 8 and warmed air in the inside is exhausted from the upper vent 9 to promote heat-dissipation in the bushings 3. Upward air flow due to convection along the surface of the heat dissipation part is accelerated in the heat dissipation part 2.