PURPOSE: To avoid the deterioration of the dielectric strength at the low temperature and maintain the dielectric strength equivalent to that in the atmosphere by a method wherein a continuity tube through which only air can pass and which is continuous to the atmosphere is provided to the top of a transformer tank.

CONSTITUTION: A continuity tube 7, which is continuous to the atmosphere, is provided to the top of a transformer tank 1. A porous filter 8 is provided to the path of the continuity tube 7. The porous filter 8 has fine holes through which molecules composing the air can penetrate but vapor molecules of condensing insulation coolant 3 such as fluorocarbon can not penetrate. The filter 8 is made of, for instance, a porous film of tetrafluoroethylene high polymer. When the vapor pressure of the fluorocarbon increases, the air is discharged as shown by an arrow 11. The air and the fluorocarbon vapor are mixed by the thermal movement but at the temperature around 90W100°C the air layer 10 is separated from the vapor phase 5 of the fluorocarbon. Therefore, the fluorocarbon vapor is condensed in the portion below a boundary plane 9 of a cooler 4 and the cooler 4 performs the cooling function in this portion.