To provide a traction device enabling a tractor to be maneuvered simply as motive power for moving backwards, by directionally steering a dolly independently of a truck frame in driving a trailer backward to the right and left on a curve, without operating the steering wheel of the tractor in directing the dolly backward.

In the trailer, the dolly rotatably provided at the front of the truck frame via a rotor is connected to the tractor by a traction bar. Using a pair of hydraulic cylinders for rotating the rotor, the dolly is freely steered for rotation about a vertical axis relative to the truck frame. At a bracket provided at the center of the front surface of a cross member at the front end of the dolly, a rotor is provided in such a way as to freely rotate about a vertical axis and a stopper is provided for engaging the rotor to non-rotatably hold the rotor or disengaging the rotor to rotatably hold the rotor. The traction bar that is long and straight is installed and connected to the rotor.