PURPOSE: To make a smooth turn achievable in a small turning radius by installing a brake, which adds a range of resistance, where a rear wheel at the inside in the turning direction at time of turning is rotatable, in this rear wheel, and keeping off a slip and a side slip in the circumferential direction of both front and rear wheels with certainty.

CONSTITUTION: Each rear wheel 21 is connected to a rear-wheel differential gear 13 being housed in a transmission case of a tractor via a brake 19 and a final reduction gear 20, and this brake 19 is installed in the rear wheel 21 at the inside in the turning direction at time of turning so as to add a range of resistance where this rear wheel 21 is rotatable. In addition, a turning center O between a front wheel and the rear wheel 21 is set more ahead than a body line A connecting the center of both symmetrical rear wheels 21, and when an angle made up of this body line A and a turning line B connecting the rear wheel 21 at the inside in the turning direction and the turning center O is set to α and another angle made up of the body line A and a turning line B' connecting the rear wheel 21 at the outside in the turning direction and the turning center O is set to β, a steering angle α' becomes to be smaller than α, while a steering angle β' of the rear wheel 21 at the outside in the turning direction is made so as to be larger than β, thus a rear-wheel steering mechanism is constituted as mentioned above.