PURPOSE: To prevent the application of excessive oil pressure to the oil-immersed electrical machinery and apparatus by constituting the device so that a section between an electrical machinery and apparatus proper and a conservator and a section between the conservator and a cooler are each circulated forcedly through the conservator.

CONSTITUTION: The conservator 11 is connected to the oil-immersed electrical machinery and apparatus 1 installed under the ground through an oil pump 8 and oil pipings 9, 10. The conservator 11 receives oil in the oil pipings at positions higher than the oil-immersed electrical machinery and apparatus 1, and has volume absorbing a voluminal change of oil due to a temperature change. The cooler 12 is installed on the roof, and a section between the cooler 12 and the conservator 11 is connected by an oil pump 13 for circulating oil and oil pipings 14. Accordingly, only head pressure by the height of the surface of a liquid of the conservator 11 is made contain during either of operation or stoppage as oil pressure applied to the oil-immersed electrical machinery and apparatus.