To allow a trailer and a tractor to display substantially integral turning behavior by arranging a driving means in a wheel of the trailer and arranging a turning locus controlling means for giving a turning locus complying with a steering angle of a steering wheel when a vehicle towing the trailer is moved backward.

When varied kinds of signals from a tractor vehicle 2 are inputted to an electronic control unit 7, a retraction time of the tractor vehicle 2 is detected from a shift position signal SP and a brake fluid pressure signal BRK, and on the basis of a vehicle speed signal V or a throttle opening signal γth, rotational speeds of left and right wheels 4L, 4R are decided. In addition, a rotational speed deference between the left and right wheels 4L, 4R for follow-up to a locus to be found is computed, and motors 5L, 5R for the left and right wheels 4L, 4R are controlled so that one wheel in a trailer 1 generates that difference. In this way, desirable locus can be followed with usual maneuvering even when a trailer is connected.