A bearing support frame (4) is disposed within a rear end portion of a tractor front housing (1) and is fixedly supported by this housing which has an internal partition wall (1a) and an open rear end. An additional speed change mechanism (12) having plural fluid-operated clutches (54F, 54R) mounted on a primary shaft (7) of a high level which extends through the partition wall is disposed within a rear half of the front housing such that it is operable to perform a speed change transmission between the primary shaft and a co-axial output shaft (10) supported by the support frame. Fluid supply passages (55F, 55R, 55L) in the primary shaft are connected to fluid supply passages (61F, 61R, 61L) in the partition wall through annular fluid chambers (60F, 60R, 60L) defined between the primary shaft and a hollow cylindrical portion (37) of the partition wall. A control valve assembly (63) supported by the front housing extends through a side wall of this housing and includes a directional control valve (78), pressure-reducing valve (83), modulating relief valve (86), and lubricant regulator valves (90, 91).