PURPOSE: To reduce the hydraulic pump driving energy for a cooling fan driving device in which a hydraulic pump for a cooling fan driving motor and a hydraulic pump for power steering are arranged in series.

CONSTITUTION: A supply passage 8 is connected in the part ranging from the first hydraulic pump 2 driven by an engine 1 to a motor 5 for driving a cooling fan. In the supply passage 8, a pressure switch 9 for detecting that the drive pressure of an actuator 6 for power steering which is driven by the second hydraulic pump 3 reaches a specified value is installed. Further, a controller 10 which receives the output signal of the pressure switch 9 and an electromagnetic control valve 4 driven by the controller 10 are installed. When the load of the actuator 6 for power steering incrases and the drive pressure reaches a specified value, a portion of the discharged oil of the first hydraulic pump 2 is supplied into the actuator 6 for power steering by selecting the electromagnetic control valve 4 from A position to B position.