This paper describes a strategy of backward motion control in a tractor-trailer system. The backward motion of a tractor-trailer requires the control skill, because a position and an orientation of a trailer must be controlled by only an input to a tractor. When a driver changes orientation in backward driving, the steering angle must be controlled skillfully. This operation is different in case of the backward motion of a single vehicle. Then the uncontrollable situation of a tractor-trailer motion often happens, for example "jackknife phenomenon." Such a situation is often induced by nonholonomic constraint of wheeled system. In this research, the virtual steering is introduced to a trailer. This virtual steering controls orientation of a trailer, and suppresses nonholonimic equivalent disturbance. So with this virtual steering, a tractor-trailer is able to track straight trajectory in backward driving independently of nonholonomic constraint. Several numerical and experimental results are shown to confirm the validity of the proposed approach.