In a front housing (1) including at a front end portion thereof an engine flywheel (6), there are provided a primary drive shaft (8) which is co-axial with the flywheel, a transmission shaft (11) which is parallel to the drive shaft, and an output shaft (13) which is co-axial with the drive shaft. The output shaft is connected to a speed change mechanism (17, 20) in a transmission case (2) succeeding to the front housing. A first supplemetary speed charge mechanism (12) is disposed between the drive shaft and the transmission shaft, and a second supplementary speed change mechanism (14) is disposed between the transmission shaft and the output shaft. One of these supplementary change mechanisms is formed into a direction-reversing mechanism, and the other change mechanism is formed into a high/low speed-selecting mechanism. Preferably, the front housing includes a first bearing support wall (1a) disposed before the first supplementary change mechanism, a second bearing support wall (4) disposed between the first and second supplementary change mechanisms, and a third bearing support wall (5) disposed behind the second supplementary change mechanism. The second and third bearing support walls are removably secured. Preferably, the second supplementary change mechanism comprises a fluid-actuated clutch (45) and a spring-actuated clutch (46) which are supplied with fluid pressure through a single fluid passage (61) in the transmission shaft.