PURPOSE: To certainly start forward and reverse stages concerning a thermal load by simultaneously loading two friction clutches of the lowest speed changing stages for starting at the time of forward and reverse traveling and maintaining a torque transmission route through them untill the same number of rotation is performed.

CONSTITUTION: An input shaft 5 and a main spindle 6 are connected to each other in a torsionally resisting state, and rotational direction reversing means 113, 114 are provided in a torque transmission route reaching driven pinions 134, 111 from the input shaft 5. The torque transmission route is maintained through a friction clutch 125 at the latest the same number of rotation is performed by the friction clutch of the lowest gear stage by providing a means to simultaneously load the friction clutches 125, 127 for starting at the time of forward traveling and the time of reverse traveling. In the above constitution, it is possible to eliminate a separation clutch between the main spindle 6 and the input shaft 5 and to certainly start at the time of a forward stage and a reverse stage concerning a thermal load of the clutch.