To adjust an engine speed to an optimum engine speed by finding a toque converter engine speed indicating desired engine speed and an optimum engine speed from a ratio of the torque converter output speed to the engine speed.

A power train 202 such as a wheel loader incorporates an impeller clutch 224 and a lock-up clutch 226 between an engine 204 and a toque converter 208. An electronic controller 232 receives output signals from an engine speed sensor 234, a toque converter speed sensor 236 and a pressure sensor 244 for measuring a force exerted to an working machine, and obtains a ration of the torque converter output speed to the engine speed so as to obtain a desired engine speed from the ratio. Further, it compares a toque time limit engine speed obtained from the torque converter speed or the like with the impeller clutch engine speed so as to obtain an optimum engine speed.