To reduce the load applied to a front wheel and prevent partial wear of tire of any wheel on rear-front axle or rear-rear axle by installing a kingpin, which fixes rear-front wheels or rear-rear wheels to the both ends of rear-front axle or rear-rear axle steerably, in such a manner that caster angle is obtained.

A kingpin 31 is structured so that it is mounted on a rear-front axle 18 to allow inclination to backward and a caster angle is obtained when travelling forward. By installing the kingpin so that rear-front wheels 17 are steerable, the rear-front axle 18 rotates with the kingpin 31 inclined backward. Therefore, the axis line of the kingpin 31 passes a point ahead of the ground- contact point of the rear-front wheel 17, which causes the same effect as pulling the rear-front wheel 17 frontward. This enables both left and right rear-front wheels 17 to self-steer by receiving the force from the road surface when front wheels are steered. Therefore, extension of rotation center of left and right rear-front wheels 17 matches a point on the extension of the rear-rear wheels as center of turning.