PURPOSE: To enable the direction change of a trailer pulled by a tractor by pivotally supporting a front end side of a pair of tracting members by means of a support lever pivotally mounted on a draw bar on the tractor side, and steering the trailer according to the relative position between the draw bar and the support lever.

CONSTITUTION: In a device where a draw bar 2 is e quipped with a traction member 3 through which a trailer 4 is connected, the member 3 is provided with a traction bar 20 and a sub-bar 21 parallel thereto. An end of the bar 20 on the side of the bar 2 is connected to a base body connected to the bar 2 by means of a support shaft 23 on which the central part of an L-shaped support lever 27 is supported. One end of the sub-bar 21 is journaled to the tip end of one of arms 27a of the lever 27 while at the tip end of the other arm 27b a displacement detecting sensor 30 is set to detect the change of the relative position between the bar 2 and the lever 27, controlling a steering mechanism 15 of the trailer 4.