Sensor-equipped hub units 34a attached to respective drive wheels 34 each have a sensor device comprising a sensor 36 for detecting the ground contact load on the wheel 34. A control unit 31 has a traction controller 32, whereby when the ground contact load value output from one of the ground contact load sensors 36 while the vehicle is traveling straight is outside a predetermined range, the drive wheel 34 is controlled so as to return the ground contact load value to the original value.