PURPOSE: To improve performance of transmission and reduce power of a hydraulic pump by controlling operating oil pressure which is supplied to a control section of transmission based on input torque of transmission which is obtained from data which is detected by a few sensors, is not affected by fluctuation of load of an auxiliary machine of engine, and is accurate.

CONSTITUTION: An engine is provided with a sensor 5 which detects the number of revolutions of engine, an output shaft 11 of a transmission 4 is provided with a sensor 9 which detects the number of revolutions of output, and the transmission is provided with a range sensor 12 and an oil temperature sensor 14. Speed ratio e of a torque converter is obtained from the number of revolutions of engine Ne, the number of revolutions of output shaft No, and gear ratio Rg which are detected by each sensor in an electronic controller 10. Further, torque ratio t and capacity coefficient C of the torque converter are obtained from speed ratio e and temperature to of operating oil. Input torque Ti of transmission is obtained from the number of revolutions of engine Ne, torque ratio t of torque converter, and capacity coefficient C. A hydraulic control valve 8 is driven based on input torque Ti of transmission, and operating oil pressure PL which is supplied to a control section of the transmission 4 is kept at adequate value.