PURPOSE: To generate synchronization of a main transmission quickly, easily, and with a low energy by installing an input shaft, auxiliary part output shaft, and 3-position type friction clutch which are coupled mechanically for performing the driving with a prime mover of a vehicle without releasing the engaged condition.

CONSTITUTION: The input shaft 22 of a transmission is mechanically coupled with an engine 12 in such a condition that engagement is not released, and an auxiliary part output shaft 46 defines an input part for a main transmission part 20 and possesses an input gear 48 of the main transmission part 20. An auxiliary transmission part 18 has a three-position type friction clutch 50 which takes an engagement position corresponding to the first and the second speed ratio and a third position to admit independent rotation of the shafts 22 and 46, and in the third position, no torque transmission takes place from the shaft 22 to the main transmission 20. In this position, a jaw clutch 66 is installed between the clutch 50 and the main transmission part 20, so that the inertia of the main transmission part 20 is decreased, and synchronization of the main transmission 20 can be established quickly, easily, and with a low energy.