PURPOSE: To eliminate a particular detection means and the like such as inserting a shaft into a mission shaft by detecting the output rotation of a torque converter by means of detection of both the rotation of a bevel gear located at the downstream of a power transmitting path and the step of gear shifting of a transmission.

CONSTITUTION: A rotation sensor 35 is disposed at a place close to the tooth surfaces of a bevel gear 26 rotatably supported by the lateral shaft of a power transmitting device, and the rotation sensor 35 is connected to an electronic controller 37 by an electric circuit 36. Meanwhile, hydraulic switches 45 through 47 are disposed at the reverse clutch, the second forward speed clutch and the third forward speed clutch of a transmission respectively, and hydraulic switches 45 through 47 are connected to the electronic controller 37 with respective electric circuits 48 through 50. And in the electronic controller 37, the reduction ratio of the bevel gear 26 to a pinion gear 26 and the like are operated so as to be processed based on the number of revolution of the lateral, shaft 25, so that the output rotation of a torque converter is thereby detected. After that, automatic control for automatic gear shifting is assumed via a control output signal circuit 51 in response to the detected output rotation.