A cooling fan drive device system (21; 21A; 21B) for a travel type working machine including a cooling fan for cooling an engine cooling water, a hydraulic pump (22; 22A) driven by an engine, a hydraulic motor (23; 23A) actuated-by a hydraulic fluid delivered by the hydraulic pump for rotating the cooling fan, and temperature detecting means for detecting the temperature of the engine cooling water, wherein said cooling fan drive system comprises: rotational speed detecting means for detecting the rotational speed of the engine; and cooling fan control means (24, 35a, 35b, 35c, 35d, 35e, 35f, 35g, 35h; 24, 35a, 35b, 35c, 35d, 35f, 35g, 35h, 35i; 44, 35a, 35b, 35c, 35d, 35e, 35f, 35h, 35j; 54, 35a, 35b, 35c, 35d, 35e, 35f, 35h, 35k) for controlling the rotational speed of the hydraulic motor on the basis of the detection values of the temperature detecting means and the rotational speed detecting means so that the rotational speed of the cooling fan is increased as the temperature of the engine cooling water rises up and the increase of the rotational speed of the cooling fan is limited when the rotational speed of the engine increases, characterized in that the rotational speed detecting means has means for detecting the target rotational speed of the engine and means for detecting the actual rotational speed of the engine, and the cooling fan control means (24; 35a, 35b, 35c, 35d, 35f, 35g, 35h, 35i) calculates the fan target rotational speed that increases as the temperature of the engine cooling water rises up, calculates the limiting value of the fan target rotational speed that is lowered as the rotational speed difference between the target rotational speed and the actual rotational speed of the engine increases, corrects the fan target rotational speed so that the fan target rotational speed does not exceed the limiting value, and controls the rotational speed of the hydraulic motor so as to achieve the corrected fan target rotational speed. rotational speed detecting means for detecting the rotational speed of the engine; and cooling fan control means (24, 35a, 35b, 35c, 35d, 35e, 35f, 35g, 35h; 24, 35a, 35b, 35c, 35d, 35f, 35g, 35h, 35i; 44, 35a, 35b, 35c, 35d, 35e, 35f, 35h, 35j; 54, 35a, 35b, 35c, 35d, 35e, 35f, 35h, 35k) for controlling the rotational speed of the hydraulic motor on the basis of the detection values of the temperature detecting means and the rotational speed detecting means so that the rotational speed of the cooling fan is increased as the temperature of the engine cooling water rises up and the increase of the rotational speed of the cooling fan is limited when the rotational speed of the engine increases, characterized in that the rotational speed detecting means has means for detecting the target rotational speed of the engine and means for detecting the actual rotational speed of the engine, and the cooling fan control means (24; 35a, 35b, 35c, 35d, 35f, 35g, 35h, 35i) calculates the fan target rotational speed that increases as the temperature of the engine cooling water rises up, calculates the limiting value of the fan target rotational speed that is lowered as the rotational speed difference between the target rotational speed and the actual rotational speed of the engine increases, corrects the fan target rotational speed so that the fan target rotational speed does not exceed the limiting value, and controls the rotational speed of the hydraulic motor so as to achieve the corrected fan target rotational speed.