PURPOSE: To improve the interchangeability and mass production performance of a clutch mechanism and obtain a small-sized and compact transmission case by juxtaposing total six main and subspeed change hydraulic clutch mechanisms, on a traveling system countershaft which penetrates through the transmission case of a tractor.

CONSTITUTION: The total six hydraulic clutch mechanisms (C1-C6) are interposed in each longitudinally symmetrical arrangement state as pairs in the contiguous part between the first main transmission hub gears (28F1) and a reverse hub gear (28R) which are arranged on a traveling system countershaft (19), in the contiguous part of the second and third main transmission hub gears (28F2) and (28F3), in the contiguous part of the first and second sub-transmission hub gears (29H) and (29L).