Abstract: An anti-skid device for use in a vehicle braking system and comprising a master cylinder for generating pressurized fluid in response to the actuation of a brake pedal, a reservoir supplying hydraulic fluid to the master cylinder, a deceleration detector for detecting the deceleration of a vehicle body the speed of which having been decreased in response to the actuation of the brake pedal, a first valve device for transmitting the fluid pressure generated in the master cylinder to a wheel brake cylinder in actuating the brake pedal and for intercepting the communication between the wheel cylinder and the master cylinder when the deceleration detector detects a predetermined amount of deceleration. The first valve device includes an expansion chamber normally filled by a predetermined amount of pressurized fluid, a dump valve communicating the expansion chamber with the reservoir when the deceleration detector detects the deceleration, and a pump device for pressurizing the expansion chamber.