PURPOSE: To reduce the radius of revolution without roughing the field by setting off the steering angle of the front wheel on the inside of revolution larger than the steering angle of the front wheel on the outside of revolution under the condition of front wheel acceleration, and meantime setting the ratio of the steering angles to a certain value which does not allow generation of a side force.

CONSTITUTION: A left and a right front wheel 10 are switchable form the standard drive condition, in which they are driven at approx. the same speed as the left and right rear wheels 6, to the front wheel acceleration condition, in which driven in higher speed than the left and right rear wheels 6, and vice versa. Switching from the standard drive condition to front wheel acceleration condition is performed with steering of the left and right front wheels 10 which exceeds the set angle. In the front wheel acceleration condition the steering angle α2 of the front wheel 10 on the inside of revolution is set off greater than the steering angle α1 of the front wheel 10 on the outside of revolution, and meantime the ratio of the steering angles α2, α1 is set to a certain value (for ex., 70°/30°) which does not allow generation of side forces F1, F2 to cause sideways slip of the front wheels 10.