PURPOSE: To miniaturize the machine body of a tractor, by setting a forward/backward variable transmission in a forward/backward gear chamber which is behind a main transmission gear chamber, and setting a sub-variable transmission in a sub- transmission gear chamber, in the tractor in which a main variable transmission is contained in the main transmission gear chamber.

CONSTITUTION: This forward/backward variable transmission is formed with a main variable transmission B adapted to get forward four-stage traveling transmission ratio, which the constituted of a group of gears for main transmission on an input main shaft 15, and and a group of gears for main transmission on the corresponding position on a a main transmission gear shaft 39. Moreover, at the rear end of the gear shaft 39, a forward . backward variable transmissin E is formed while said transmission E being constituted of both forward/backward synchro gear 69, 70 and a selector sleeve 71, forward/backward shuttle gear 65, 66 set on the corresponding position on a shuttle gear shaft 63, and idle gear 68 on an idle gear shaft 67. Accordingly, a tractor machine body can be driven forward according to this transmission gear ratio obtained by the main variable transmission B, by sliding the sleeve 71 forward by lever operation in the direction of an arrow F. At this time, various transmitting operations are performed by the a sub-variable transmission D.