To lessen a shift load by providing an auxiliary clutch mechanism which separates a clutch from a speed change gear and allows a shift operation irrespective of the inertia of the clutch.

An auxiliary clutch mechanism 94 which is operated in response to the state of pressing of a clutch pedal 136 is disposed between an input shat 8 and an intermediate shaft 10. If the clutch pedal 136 is pressed and a shift control lever 92 is operated, the clutch plate of a clutch 6 is completely separated from a speed change gear 2 by a dog clutch 98 and a shift operation can be performed irrespective of the inertia of the clutch 6. Next, if the clutch pedal 136 is returned, the clutch 6 is connected to the speed change gear 2 by the dog clutch 98. Moreover, if the clutch pedal 136 is returned, the clutch 6 is completely connected and a speed change is completed. Accordingly, when a speed change is performed, the shift operation can be performed irrespective of the inertia of the clutch and a shift load can be lessened.