<p>A sensor-equipped hub unit (34a) mounted on each drive vehicle (34) has a sensor device having a grounding load sensor (36) for measuring a grounding load of a wheel (34). A control device (31) has a traction control section (32). When a grounding load value output from any of the grounding load sensors (36) deviates from a predetermined range while the vehicle is running straight, the drive wheels (34) are controlled by the control device (31) so as to return the grounding load value to a previous value.</p>
