In a cooling fan drive system for a travel type working machine, the rotational speed of a cooling fan is controlled to the optimum rotational speed in accordance with temperature increase of the temperature of an engine cooling water, and the engine rotational speed is smoothly increased when the engine rotational speed is increased under travel acceleration. A fourth fan target rotational speed calculator 35e and a minimum value selector 35f are provided, and under non-operation, the fan target rotational speed is set to a low rotational speed irrespective of the temperature. When an acceleration pedal 12 is depressed to increase the rotational speed of the engine under travel acceleration, the increase of the drive pressure of a hydraulic motor 23 due to increase of the rotation of the cooling fan 9 is suppressed, and the load on the engine 1 is reduced.