First rear swing arms are pivotably supported at their front ends to a drive shaft on which drive wheels are mounted. The rear ends of the first rear swing arms are pivotably supported through a shaft to the front ends of side rails, and first idlers are mounted on the shaft. Second idlers are mounted on a shaft to which the rear ends of the side rails are supported, and second rear swing arms are supported at their lower ends to shafts supported to the rear ends of the side rails. The upper ends of the second rear swing arms are supported to a shaft supported to a floor. A track belt is wrapped around the drive wheels and the idlers. Sliders and are individually mounted on the first rear swing arms and the side rails, respectively.