PURPOSE: To detect and prevent a trailer swing condition and a jackknife condition in an initial stage by determining a tractor-trailer articulation angle, and a primary and a secondary derivatives of the articulation angle, and processing these in accordance with specified logical rules, and controlling an ABC valve.

CONSTITUTION: A tractor 12 and a trailer 14 are articulated to each other by engagement of a fifth wheel 34 with a trailer 14 side king pin 176, a tractor brake system is provided so that pneumatic brakes 60, 62, 64, 66, 152, and 154 brake rotation of wheels, and a front control valve 108 and a rear control valve 168 are controlled by a central processing unit 70. In this case, one or plural sensors 170, 172 are provided on the fifth wheel 34 or the like, input signals expressing a current articulation angle (AA) and its time derivatives such as a primary derivative (change ratio) (dAA/dt) of the articulation angle (AA) and a secondary derivative (d2AA/dt2) are sent to the central processing unit 70, where they are logically processed to generate a command output signal, thereby a control valve 174 is controlled.