PURPOSE: To quickly detect the transit point to the road surface at a high so as to eliminate the shortage of the braking force by detecting the transit of a vehicle to the road surface at a low μ after passing the predetermined time as a total in the condition that the wheel speed of both front wheels coincide with the approximate car body speed and slip factor of both rear wheels exceeds the predetermined slip factor.

CONSTITUTION: When a brake pedal 4 is operated, a master cylinder generates the hydraulics in two hydraulics generating chambers, and the hydraulics of each chamber is supplied to each wheel cylinder of a left front wheel FL and a right rear wheel RR: a right front wheel FR and a left rear wheel RL through inlet valves 17a, 17b:20a, 20b. In this case, the wheel speed of both the front wheels FL, FR is always observed, and compared with the approximate car body speed. Slip factor of both the rear wheels RL, RR is always detected. In the case where the slip factor of the rear wheels is larger than the predetermined value, a judgment that both the front wheels move to the road surface at a high μ is performed in the condition that both the rear wheels exist on the road surface at a low μ but that the front wheel speed coincides with the predetermined value, and the braking force is controlled for increase.