PURPOSE: To perform smooth and speedy speed change control from a current speed change stage to a next speed change stage by performing such control as to start release action of a next speed change stage synchro-clutch means by a speed change actuator when an input side engine speed of the next speed change stage synchro-clutch is synchronized with an output side engine speed.

CONSTITUTION: When a current speed change stage synchro-clutch means is released to be neutral, an input side engine speed of a next speed change stage synchro-clutch is synchronized with an output side engine speed, and control of maintaining a neutral condition is performed by a speed change actuator A2. After this, when the input side engine speed of the next speed change synchro-clutch is synchronized with the output side engine speed, control of starting engagement action of the next speed change stage synchro-clutch means is performed by the speed change actuator A2. As a result, smooth and speedy speed change control can be performed from a current speed change stage to a next speed change stage.