PURPOSE: To make it possible to apply brake forces to right and left wheels of a trailer, respectively, in accordance with the rotational speeds of these wheels, by coupling the right and left wheels through the intermediary of a differential gear unit, and by providing a brake mechanism to the rotary shaft of a drive pinion in this differential gear unit.

CONSTITUTION: In such an arrangement that a trailer 2 is coupled, through the intermediary of a coupler 21, to the rear section of a tractor 1 having left and right wheels 12 to which a torque is transmitted from an engine 11 through a transmission 13 and a differential gear unit 14, left and right wheels 22 of the trailer are coupled together through the intermediary of a differential gear unit 23 having a drive pinion whose shaft is attached thereto with a brake mechanism 24 adapted to be operated in association with a brake mechanism in the tractor 1. With this arrangement, when the tractor 1 is retarded by braking, the brake mechanism 24 retards the rotation of the drive pinion shaft in the differential gear unit 23, and accordingly, braking force are transmitted respectively to the wheels 22 through the inside gear mechanism in accordance with the rotational speeds of the wheels 22.