PURPOSE: To prevent a field from being damaged by a wheel at turning time while a tractor can be turned or a small radius by providing a grounding unit, so that it can be lifted further relatively turned about the axial center of vertical direction relating to the tractor, on a lower surface thereof.

CONSTITUTION: In the case of turning a tractor 1 in an end part or the like of a field, a grounding unit 6 is lowered down and grounded to the ground surface to float the tractor 1 from the ground surface. In this condition, when the grounding unit 6 is relatively rotated about the axial center of vertical direction relating to the tractor 1, since the grounding unit 6 is grounded to the ground surface, the tractor 1 is turned about the center of rotation of the grounding unit 6. In this way, the tractor 1 can be turned on a small radius, and further the field is prevented from being damaged by a wheel of the tractor 1.