PURPOSE: To contrive improvement in reliability of the titled device as well as to increase in its cooling efficiency by a method wherein a branching device, with which the oil contained in a cooler is discharged through a pump, is provided at the coupled part which is used to couple the cooler and a tank.

CONSTITUTION: A branching device, having a large-calibered coupling pipe 71 arranged between a tank 3 and a cooler 4 and a small-calibered pipe 72 with which the oil fed from a pump 5 through a hole provided on the coupling pipe 71, are provided. The oil fed from the pump 5 is delivered from the delivery pipe 72 of the branching device as shown by the solid line arrow in the diagram when the pump 5 is in operation. Then, when the pump 5 is in a non-operated state, the oil runs between the outer circumference of the delivery pipe 72 and the coupling pipe 71 in the direction as shown by the dotted line arrow in the diagram. As a check valve device is not used as above-mentioned, the wasteful space in height direction can be removed, the height of the cooler 4 can be increased, and the trouble of a transformer caused by an unsatisfactory valve device can be eliminated, thereby enabling to improve the reliability of the transformer.