PURPOSE: To automatically prevent contact of the corner part of a car body with an obstacle when the rear part of the car body is laterally moved through rotation Of an auxiliary wheel.

CONSTITUTION: An auxiliary wheel 11, the rotary shaft of which is extended in the longitudinal direction of a vehicle is located on the under side of the rear of a car body. When an auxiliary wheel actuating switch 42 is operated, valves 25 and 27 are changed over and pressure oil is fed to the upper oil chamber of a hydraulic cylinder 13. Through the feed of the pressure oil, the hydraulic cylinder 13 lowers the auxiliary wheel 11 to lift a car body. When, in this state, a rotation indication switch 43 is operated, valves 31 and 32 are changed over and the auxiliary wheel 11 is rotated with the aid of a motor 15, whereby the rear part of the car body is laterally moved. Meanwhile, when approach sensors 47a-47d located to the corner part of a car body detect approach of the corner part to an obstacle, the valve 32 is restored to an original state (Fig 1) and rotation of the auxiliary wheel 11 is prohibited by the motor 15, whereby lateral movement of the rear part of the car body is stopped.