PURPOSE: To enhance the safety of a brake system upon failure of an anti-skid device, by dipsosing a stopper pin in a small bore opend to a slidable piston hole in the anti-skid device so that the pin is arranged to be projected by spring force upon failure of a control hydraulic pressure system to be engaged with a piston.

CONSTITUTION: Hydraulic pressure which is produced from a master cylinder 45 by operating a brake pedal 53, acts a brake 47 through a valve hole 10 to carry out normal brake operation. At this time, if the locking condition of a wheel 46 due to abrupt braking is detected from the output of a rotational speed sensor 48, a controller 49 opens a solenoid valve 37. Thereby, a piston 22 moves to the left, overcoming a spring 35, and therefore, a ball valve 12 closes a valve hole 10. Accordingly, brake hydraulic pressure is cut off to release the brake 47. Further, if a control hydraulic pressure system is failed due to failure of a hydraulic pump 51, a stopper pin 38 is projected by a spring 41 to engage with the stepped part 42 of a piston 22 for nullifying the function of the anti-skid device, thereby the normal function of braking may be ensured.