To generate braking force at proper timing automatically when a trailer needs it by providing detection means which detects that a king pin presses a coupler side and a controller which controls in such a manner that a retarder is operated by the detection means.

When a semitrailer 12 pushes a tractor forward in such a manner that it raises the tractor from behind, a king pin 20 moves forward on a coupler and is energized to the left so that a switch 64 is changed over from an OFF condition to an ON condition. A detection signal of the switch 64 and an output detected by a back gear sensor 75 of a transmission of the tractor are input into a controller 80. When the switch 64 is changed over to ON while the back gear sensor 75 is in an OFF condition, the controller 80 conducts electrification into a retarder 28, and the retarder 28 generates braking force to give the braking force to front and rear wheels 18 of the semitrailer 12. Consequently, it is possible to ensure the stability of running at the time of deceleration.