PURPOSE: To improve steering stability ever so better, by estimating the skid angle information that is required for controlling a skid angle so as to be set to the specified value on the basis of a car model, while steering and controlling a car according to a deviation between the estimated skid angle and the desired skid one.

CONSTITUTION: At the time of controlling a rear-wheel steering mechanism 6 according to the steerage of front wheels by operation of a steering wheel, each of detection data δf, δr, M and γ of front-rear wheel steering angle sensors 21 and 26, a car speed sensor 23, a car weight sensor 24 and a car yaw rate sensor 27 is inputted into a state observer 28. In this state observer 28, the moment of car's yaw inertia, a point of the center of gravity in a car, length between front-rear wheel axles and front-rear tire cornering power are stored in regard to two-degree of freedom models in a two-wheeled vehicle. Hereat, according to the said input data, the estimated value of skid angle β is outputted. Then, subsequently a servo valve 17 for rear-wheel steering use is controlled according to a deviation between this estimated skid angle β and a desired skid angle β0.