Actual temperatures of intake air, hydraulic fluid, and coolant being cooled by means of a cooling fan are detected, target numbers of revolution (Nti, Nto, Ntc) of the fan are determined by PI controllers (37-39) depending on the temperature difference between the actual temperatures (Tmi, Tmo, Tmc) and target temperatures (Tti, Tto, Ttc), and then the cooling fan is controlled by the target numbers of revolution (Nti, Nto, Ntc) of the fan. In order to limit accumulation for the negative side of integration in the PI controllers (37, 38, 39), an integration start control means (41) for controlling the timing of starting integration is provided for the PI controllers (37, 38, 39). Target temperatures (Tti, Tto, Ttc) for starting integration are set, and the integration elements of the PI controllers (37, 38, 39) are arranged to function only after the actual temperature (Tmi, Tmo, Tmc) of intake air, hydraulic fluid, or coolant reaches the target temperature (Tti, Tto, Ttc) thus preventing rising response lag of the number of revolution of the fan.
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