When wheel speed is significantly reduced from estimated vehicle speed, the VSA modulator-control unit closes the inlet valve and opens the outlet valve momentarily to reduce brake fluid pressure and the pump motor starts to run. As wheel speed increases, the VSA modulator-control unit maintains brake fluid pressure by keeping the inlet valve closed. Brake fluid pressure is increased when wheel speed matches the estimated speed of the vehicle. The VSA modulator-control unit repeats this valve control operation to control the ABS function.