An actuator is disclosed for use in an anti-skid brake system for wheeled vehicles. A first valve is disposed in a pressure line to selectively divide the pressure line into two separate lines. Also included is an electric motor, a pump driven by the electric motor, an accumulator adapted to accumulate fluid pressure generated by the pump, a control piston reciprocating under the fluid pressure to open and close the first valve, a second valve adapted to supply the fluid pressure to the control piston upon normal rolling of wheels so as to move the control piston to a non-operative position while excluding the fluid pressure from the control piston when the wheels are likely to lock up, and a third valve having first and second valves and controlling the fluid pressure to be supplied to a hydraulic chamber in response to depression and release of a brake pedal. In this arrangement, a relief valve is provided in a seat member of the first valve so that when the fluid pressure exceeds a predetermined upper limit, it is opened to introduce hydraulic fluid to a drain line through the second valve.