PURPOSE: To vary the rotational speed of right and left hydraulic motors by changing the discharge thereof with the shifting of the link ratio through operation of a control lever for controlling a link between a discharge controlling lever of a variable discharge type hydraulic pump and an accelerator.

CONSTITUTION: Variable discharge type hydraulic pumps 3 and 4 are connected to a prime mover 1 by way of a decelerator 2. The pumps 3 and 4 are connected to hydraulic motors 6 and 7 whose shaft output is connected to right and left wheels 8 and 9. Links 16 and 17 are connected to the tip of a lever 14 integral with an acceleration lever 10, to which control levers 19, 21 of the hydraulic pumps are connected while push-pull cables 26 and 27 of a control lever 25 for the front wheel steering are connected. With the operation of the control lever 25 to the right, the wheel turned to the left through a tie rod 24 and simultaneously, the link 17 turns and the link 16 to the left by way of the cables 26 and 27. The angle of the levers 19 and 21 on the hydraulic pumps 3 and 4, the motor 6 leading to the pump 3 rotates at a constant speed and the motor 7 at a reduced speed.