To estimate a state of a road face highly accurately by estimating a friction coefficient of the road face, comparing the coefficient with a previously estimated value, and controlling not to update the coefficient to a lower value from the previously estimated value based on the comparison result when a driving wheel slips not larger than a predetermined value.

Velocities of driving wheels 14, 16 and driven wheels 10, 12 are measured by driving wheel velocity sensors 48, 50 and driven wheel velocity sensors 52, 54, which are input to an electronic control device 46. The device 46 obtains a friction coefficient of a road face from a previous velocity of the driven wheels and the present velocity of the driven wheels, and compares the coefficient with a previous value. When a present value of the friction coefficient is larger than or equal to the previous value, the friction coefficient is updated to the present value. When the present value is smaller than the previous value, it is detected whether or not a slip of the driving wheels 14, 16 obtained from a difference of velocities of the driving wheels and driven wheels is smaller than a predetermined value. If the difference is not smaller than the predetermined value, the friction coefficient is updated to the present value. If the difference is smaller than the predetermined value, the previous value is maintained. Accordingly, the friction coefficient of the road face can be estimated more highly accurately.