PURPOSE: To enable stationary attitude control of a trailer in accelerating and decelerating by making generative force in the direction to shorten the distance between an axle and a frame in decelerating act on the front part and making generative force in the direction to widen it act on rear active suspensions, and making force in the opposite direction act in accelerating.

CONSTITUTION: A tractor 2 is connected to a trailer 3 by a coupler 4. The expression; F=(maα.h + Fc.h)L.. is satisfied by taking mass of the tractor as (m), acceleration in decelerating as α, the height from the center of gravity position of the tractor 2 to the front wheel ground contact surface as h1, inertia force of the trailer, to act on the coupler as Fc, the height from the position of the coupler 4 to the rear wheel ground contact surface as h2, and the distance between the front and rear wheels and the ground contact surface as L. The attitude of the tractor 2 can be maintained into the stationary attitude by making generative force F in the direction to shorten the distance between the front axle and the frame act on front actuators AC, and also making generative force F in the direction to widen the distance between the rear axle and the frame act on the rear actuators AC.