PURPOSE: To obtain turning performance of always optimum steering characteristics by providing a road surface friction factor estimating means only for estimating road surface friction factors and changing the gain of steering angle rate correction characteristics to drive wheel slipping quantity according to the estimated road surface friction factor.

CONSTITUTION: A 4 wheel steering controller has a rear wheel steering mechanism (a) capable of steering the rear wheels and determines the steering quantity according to the steering state of the front wheels. In this case, a drive wheel slip detecting means (b) and a road surface friction factor estimating means (c) for estimating road surface friction factors are provided. In the turning which may cause drive wheel slip, a steering angle rate determining rear wheel steering quantity relating to the steering state of the front wheel is corrected by the steering angle correction quantity based on drive wheel slipping quantity and the characteristics of the gain, which becomes smaller, the lower friction factor the road surface friction factor estimation value out of the characteristics of the steering rate correction quantity to drive wheel slipping quantity shows in the rear wheel steering controlling means (d). The rear wheel steering quantity is determined by the correction steering rate after this correction.