PURPOSE: To keep the speed of a cooling fan constant by compensating a variation in the number of rotations of the cooling fan caused by lowering of volumetric efficiency of a hydraulic motor due to rise in oil temperature.

CONSTITUTION: A constant volume of controlled flow from a hydraulic pump 1 is fed to a hydraulic motor 3 through a flow control valve 2 with pressure compensation. Signals are transmitted as required when the rotational speed lowers below a preset limit to the flow control valve 2 by a rotational speed detector 7 provided on a cooling fan 4, and the hydraulic pump 1 is controlled variably to increase the amount of flow.