PURPOSE: To accomplish the rapid detection of a slip by constituting a device so that the rotating speed of a driving wheel can be obtained based on a component including a comparatively high frequency and the rotating speed of a driven wheel can be obtained based on the component of a comparatively low frequency alone.

CONSTITUTION: The rotating speed of the driving wheel is calculated as the mean value of the rotating speed of the driving wheel by a prescribed formula. Besides, the rotating speed of the driven wheel is calculated as the mean value of the rotating speed of the driven wheel by other formula. The calculated rotating speed of the driving wheel is made to be equivalent to what is obtained based on the component including the comparatively high frequency of detection signals S3 and S4 and being that frequency or under, while the calculated rotating speed of the driven wheel is made to be equivalent to what is obtained based on the comparatively low frequency of detection signals S1 and S2 alone. A control unit 100 subtracts the rotating speed of the driven wheel from the calculated rotating speed of the driving wheel, judges that rear wheels 3 and 4 slip on a road surface when the obtained difference of the rotating speed becomes a prescribed value or more and sets the target opening of a throttle for restraining and controlling the slip which is smaller than the ordinary target opening of the throttle according to the difference of the rotating speed.