PURPOSE: To simplify actions by driving rear wheels with a hydraulic motor, and switching the selector valves feeding the oil pressure to steering cylinders turning the front wheels and rear wheels respectively and the hydraulic motor with action signals of an operation lever and a turn knob.

CONSTITUTION: When an operation lever 31 is tilted forward or backward, the first encoder 35 outputs a forward or reverse signal in response to the tilt quantity, a controller 30 excites the solenoids 22a, 23a or 22b, 23b of the third and fourth electromagnetic selector valves 22, 23 and feeds the oil pressure in the positive or reverse direction to a hydraulic motor 9, and a vehicle is advanced or reversed at the speed corresponding to the tilt quantity. When the operation lever 31 is tilted to the right or left, selector valves 20, 21 are likewise operated by the second encoder 36, the front and rear steering cylinders 7, 14 are operated, and the vehicle is turned to the right or left. When a turn knob 32 is turned to the right or left, the selector valves 20, 21 are operated by the signal of an encoder 37, and the front wheels and rear wheels 3, 10 are turned to the preset directions.