PURPOSE: To avoid being oblivious of coupling operation on symmetrical brakes and thereby improve safety in tractor driving, by making a hydraulic control valve so as to be restored to the original position when symmetrical steering brake pedals are locked by hand, after these symmetrical brakes are automatically coupled through the hydraulic control valve.

CONSTITUTION: Rotation in an engine E is transmitted to an input shaft 14 via a main clutch, while an interval between the input shaft 14 and a counter shaft 15 is shifted into four-step gears. Rotation after this gearshifting and rotation in a pinion shaft both are transmitted to a differential gear 23 and then these rotations receive braking action by symmetrical braking devices 24L and 24R installed on a differential side gear shaft. When an operator puts a mechanical coupling piece 4 therein, speed goes up whereby a hydraulic control valve 9 is no longer required to operate, and a switch 28 is disconnected, so a solenoid 9a will not operate any longer. Only when the operator is oblivious of coupling operation on the coupling piece 4, if the speed goes up beyond the setting value, the solenoid 9a operates, thus the hydraulic control value 9 is opened.