To improve cooling efficiency by sufficiently ventilating an oil cooler by constituting an automatic transmission to ventilate the oil cooler by connecting a hydraulic motor to a hydraulic pump built in the automatic transmission and rotating a fan by this hydraulic motor.

An automatic transmission 11 is connected to an engine E to be loaded on a truck, but a hydraulic pump 13 to circulate oil by driving force of the engine E is built in this automatic transmission 11, and an oil cooler 19 to cool oil and a fan 21 on its side are arranged to face against each other. Additionally, a hydraulic motor 23 to be driven by oil is connected to this fan 21, and a flow rate adjustment valve 25 to adjust supply quantity of oil to the hydraulic motor 23 in accordance with an electric current value of a flow rate signal to be output from a control circuit 35 is arranged between a relief valve 15 and the hydraulic motor 23. Consequently, air quantity to the oil cooler is adjusted by rotating the fan by the number of rotation in correspondence with oil temperature.