PURPOSE: To enable the steering of rear wheels of a trailer so as to cause the angle of lateral sliding to become zero, by regarding a tractor as constituting an imaginary front wheel of the trailer and thereby steering the rear wheels of the trailer in accordance with an actual steer angle of the imaginary front wheels as viewed with respect to the trailer.

CONSTITUTION: The semi-trailer vehicle is composed of a tractor 10 and a trailer 11, the trailer 11 having no wheel at its front end portion and this front end portion being supported by a coupler at the rear end portion of the tractor 10. And the tractor 10 is provided with steerable front wheels 13 and rear drive wheels 14 while the trailer 11 is provided with rearward front wheels 15 and rearward rear wheels 16. In this case, the tractor 10 is regarded as a single large imaginary front wheel 20. A rearward front wheel shaft 17 and a rearward rear wheel shaft 18 are revolved in accordance with an actual angle of steer of the imaginary front wheels 20, thereby steering the rear wheels. Further, at this time, the actual steering angular velocity and vehicle speed as well as the actual steer angle of the imaginary front wheel are used as control parameters to calculate a steer angle for the rear wheels, thereby controlling an oil pressure cylinder for steering the rear wheels.