After a speed change operation mechanism has changed gear position during a shift in a vehicle, a controller operates an actuator to put a clutch into a partial clutch engagement state, thereby reducing difference rotation Nd of the clutch, and temporarily lowers an output of an engine below a normal-time output. After the difference rotation of the clutch is reduced, the controller returns the engine output to the normal-time output, and operates the actuator to bring the clutch into a full capacity state. The standard difference rotational speed Nds is variable according to the throttle position Th detected by the engine load detection means.