PURPOSE: To contrive to simplify the structure of a transmission case and the like by a structure wherein a countershaft is arranged laterally and a drive gear section is provided at one side of the transmission case.

CONSTITUTION: An engine output shaft 18 is situated at one side of a transmission case 9a and provided with a drive gear mechanism A, while a driven gear mechanism B is provided at the intermediate position of a wheel axle 13. Furthermore said gear mechanisms A and B are coupled to each other by means of a diagonally-arranged drive shaft system 23. The shaft system 23 and the driven gear mechanism B are situated between frames 2 in order to transmit power at the middle position of wheels. Consequently, the simplification of the structure of the cover of gear part and of the transmission case and the like is contrived and at the same time favorable power transmission is obtained.