In an anti-skid control system for a motor vehicle there is provided a fail-safe control system for providing a compensation for a proper vehicle braking operation against the occurrence of failure of a hydraulic pressure control valve. The fail-safe control system includes a device for monitoring a control signal for controlling the operation of the control valve so that the hydraulic pressure applied to a wheel braking cylinder is regulated for preventing a wheel of the vehicle from skidding. The monitoring device detects the occurrence of failure of the control valve when the control signal indicative of one of the operation modes of the control valve is continuously generated more than a predetermined time period. In response to the detection, the fail-safe control system switches a directional control valve to the passage-opening condition so that the hydraulic pressure from a master cylinder can directly be applied to the wheel braking cylinder, thereby enabling the braking operation to be controlled by a vehicle driver.
