To provide an auxiliary device for turning a lawn-mowing vehicle enabling the mowing vehicle to make a U-turn in the minimum radius without damaging lawn.

When a lawn-mowing vehicle makes a U-turn, the rod 41 of a hydraulic cylinder 37 is projected to bring a grounding means 42 into contact with the ground surface G so that a wheel 7L nearest to the turning center P is allowed to float from the ground. While the wheel 7L is allowed to float from the ground surface, the U-turn is made around the grounding means 42 which is the same axis as that of the turning center P for the U-turn. Thus, the lawn-mowing vehicle can make a U-turn without giving damage to the lawn due to slipping of the grounding part of the wheel 7L.