Abstract: When a master cylinder pressure detected by way of a pressure sensor (74) attains a level not lower than a target pressure (P.sub.OK), normally-open on-off valves (62, 63) for by-passing proportioning control valves (57.sub.1, 57.sub.2) are opened. If the slip factor of rear wheels, calculated by way of a slip factor calculation section (121), is not lower than a predetermined value, the on-off valves (64, 66) are opened to allow the rear wheel cylinder pressure to escape to accumulators (65, 67). Thus, if there is enough room for the increase of the braking force on the rear wheels, the rear wheel braking force can be increased to reduce the share of the front wheel braking force, depending on braking conditions, such as the road surface conditions, vehicle running conditions, etc. Further the rear wheels can be prevented from being locked.