PURPOSE: To prevent trailer swing by sensing and computing a value expressing at least one of an articulation angle of an articulated vehicle and a time derivative of the articulation angle, and processing the value in accordance with specified logical rules.

CONSTITUTION: A tractor-semi-trailer device 10 comprises a tractor 12 and a semi-trailer 14 articulated by an articulation means 16 of a tractor side fifth wheel 34 selectively engaged with a trailer side king pin, and pneumatic rear brakes 60, 62, 152, 154, and front brakes 64, 66 are provided. In this case, plural sensors 170, 172 are installed on the fifth wheel 34 or the like to conduct anti- trailer swing control. Input signals expressing a current articulation angle AA and its time derivative are sent to a CPU 70, a control valve 174 is controlled, and pilot or control is sent to a control valve of a trailer brake system.