To improve operability capable of easy turning without damaging a rice paddy field.

This machine is constructed such that shifted power is transmitted to right and left front wheels 1 capable of steering and right and left rear wheels 2 not capable of steering. The machine is provided with friction type side clutches 96 corresponding to the right and left rear wheels 2, respectively. A machine body can be steered only by steering of the front wheels 1 and OFF operation of one of the side clutches 96. A side clutch automatic operation mechanism 106 is also provided in this machine to implement OFF operation of the one of the side clutches 96 positioned at the inner side of turning by which the front wheels are steered at an angle not less than a set standard angle.