When a road surface gradient is calculated by a wheel driving force and a vehicle acceleration, if the wheel driving force is calculated based on an output torque of an engine (1), the accuracy of calculating the wheel driving force, i.e., the road surface gradient becomes poor due to deterioration by aging of the engine or the like. In order to solve this kind of disadvantage, a fluid transmission torque (TQtr) of a torque converter (2) is calculated by those torque ratio (ê) of the torque converter (2) and an input shaft torque coefficient (ô) which correspond to a speed ratio (etr) of the torque converter (2), and an input rotational speed (Ne) of the torque converter (2) (= engine rotational speed (step S21-3). The road surface gradient is calculated by using a wheel driving force to be calculated by this torque (TQtr).