PURPOSE: To improve stability of a vehicle by connecting proportioning valves with check valves in parallel, and keeping the split points of the proportioning valves to be set on appropriate hydraulic pressure level at sudden braking on the road of high friction coefficient.

CONSTITUTION: An antilock brake system is provided with a first hydraulic pressure passage 9 communicating a right front wheel cylinder 6b and a left rear wheel cylinder 4a each other, and a second hydraulic pressure passage 10 communicating a left front wheel cylinder 6a and a right rear wheel cylinder 4b, and a ABS actuator is provided on the way of the passage communicating a master cylinder 2, the first and second hydraulic pressure passages 9,10. Further, proportioning valves 13a, 13b are respectively provided on the rear wheel side of the hydraulic pressure passages 9, 10. In this case, check valves 14a, 14b to only allow brake oil to flow from left and right rear wheel clinders 4a, 4b to the ABS actuator 5 are respectively provided in parallel with the proportioning valves 13a, 13b.