PURPOSE: To prevent the frequent variations in a speed of a motor for a cooler without losing the loss reducing effect by composed the cooler and a variable frequency power source of at least two or more units, and setting the controlling conditions of the rotating speed to the conditions of at least one unit different from the other unit.

CONSTITUTION: A cooler 2 (2a, 2b) which forcibly feed by a motor cooling medium necessary to cool a transformer is provided, the operating state of the transformer is detected by a detector 5, and the output frequency of a variable power source 11 (11a, 11b) is controlled by a controller 12 on the basis of a signal detected by the detector 5. In such a cooler for the transformer, the coolers 2a, 2b and the power sources 11a, 11b are composed at least two or more units, the correspondence between the signal level detected by the detector 5 and the output frequencies of the power sources 11a, 11b is stepwisely set as the controlling conditions of the power sources 11a, 11b, and at least one unit of the all units of the power sources is set to the controlling conditions different from the other unit.