PURPOSE: To enable the locking of vehicle wheel at a time of sudden braking to be prevented regardless of a road-surface condition or a tire condition or the like by obtaining a car-body speed at a braking time through adjusting the increase and decrease portion of a car-body speed to an information from a vehicle wheel speed sensor at a non-braking time.
CONSTITUTION: At a non-braking time, a true car-body speed is operated through a signal from a vehicle wheel speed sensor, and a current acceleration speed is calculated by a signal from an acceleration sensor 18, and each speed is stored in a controller 7. On the other hand, at a braking time, from the acceleration of a car-body obtained from the acceleration sensor 18 is calculated the increase and decrease portion of a car-body speed, which is adjusted to the car-body speed stored previously and a current car-body speed is obtained. And a current slip ratio is calculated from the current car-body speed and the car-body speed calculated by a signal from the vehicle wheel speed sensor. This slip ratio is supplied from a controller 7 so as to control electromagnetic valves 5-7. As aforesaid, the current car-body speed is decided from the acceleration of the car-body varying according to a condition, which is supplied to operate the slip ratio. Therefore, an accurate slip ratio may be obtain at all times.