To prevent the bulging out of the rear end of a trailer to the outside at a curve by steering the rear wheels of the trailer by an angle proportional to a current connection angle in a direction reverse to the steering direction of its front wheels when a traveling distance reaches a specified distance.

After a controller 20 identifies the start of curving of a tractor 1 and a trailer 2, a traveling distance sensor 7 starts measuring the traveling distance of the tractor 1. A signal for a traveling distance measured by the traveling distance sensor 7 is inputted to the controller 20. Then, the controller 20 makes comparison to determine equality between the traveling distance and a trailer wheel base distance L. If equality is determined, a signal for a connection angle θc detected by a connection angle sensor 3a is inputted again to the controller 20. Then, based on a rear wheel steering angle θr, a steering gear 12 is actuated and the rear wheels 11 of the trailer 2 are steered.