PURPOSE: To prevent the application of excessive oil pressure to the oil immersed electrical machinery and apparatus by directly coupling an oil circulating pump and a decompression mechanism, converting the potential energy of oil into pump action and absorbing oil pressure.

CONSTITUTION: Motor valves 11, 12 are closed during the suspension of the operation of the machinery and apparatus, and oil pressure applied to an oil-immersed electrical machinery and apparatus proper 1 is the static pressure of the oil pressure of a conservator 1a. When starting operation, an auxiliary pump 10 for circulating oil is started, and the motor valves 11, 12 are opened slowly. When oil begins to circulate, the decompression mechanism 9 and the oil circulating pump 8 are operated, and brought to a normal operation state. Oil pressure applied to the proper 1 is the same as one during the suspension of the machinery and apparatus at that time. When stopping the machinery and apparatus being operated, motor valves 6, 7 are closed slowly, the oil circulating pump 8 and the decompression mechanism 9 stop automatically when the circulation of oil stops, and a voluminal change of oil with a temperature change is absorbed by each conservator 1a, 3a when the auxiliary pump 10 is stopped and the machinery and apparatus is stopped.