PURPOSE: To prevent each component from damaging by force based on vertical rocking of a housing, by fixing a steering driving mechanism moving a tie rod for steering of a front wheel to a front axle housing.

CONSTITUTION: A steering driving mechanism 18 is fixed to a front axle housing 4 hinged by a car body frame 1 and a tie rod for front wheel steering is moved by the steering driving mechanism 18. As the steering driving mechanism 18, therefore, rocks vertically along with the tie rod 16 unitarily when the front axle housing 4 is rocked vertically to the car body frame 1, rocking force does not work upon a connecting part between the steering driving mechanism 18 and the tie rod 16. The foregoing connecting part, therefore, does not break.