When a speed ratio, which is the ratio of the turbine speed to the engine speed, is equal to or less than 1, a driving torque converter dynamic characteristic model is used which is set based on a driving torque converter static characteristic in which the torque ratio decreases to 1 with an increase of the speed ratio. When the speed ratio is greater than 1, a driven torque converter dynamic characteristic model is used which is set based on a driven torque converter static characteristic in which the torque ratio is 1 regardless of the speed ratio.