PURPOSE: To improve steering stability in a combination of vehicles by steering the rear wheels of a trailer so that a lateral sliding angle at the center of gravity of the trailer may be zero (0).

CONSTITUTION: An axle housing 16 is turned counterclockwise around the ball joint mounted at the V-shaped top of an upper radius rod 40 so that the rear wheels 18 of a trailer are steered counterclockwise. When a cylinder 30 is operated in a shrinking manner, the housing 16 is turned clockwise to enable the rear wheels 18 to be turned clockwise. The wheels 18 are steered so that the lateral sliding angle at the center of gravity of trailer may be zero (0). Unnecessary yawing of trailer is restrained and also that of tractor is restrained to improve the steering stability of combination of vehicles.