PURPOSE: To enable to change the wheel base in accordance with the kind of service by a method wherein a rear axle is longitudinally adjustable.

CONSTITUTION: The rear axle 7 is pivotally supported by a bracket 23 so as to be able to be fixed forwardly and backwardly. When the rear axle 7 is fixed backwardly, an input shaft 9 is connected to a transmission shaft 16. When the rear axle 7 is fixed forwardly, a power-take-off shaft 8 is directly connected to a transmission shaft 17. The change of the wheel base makes both the service with good stability and the service requiring a small turn possible.