Disclosed is a master cylinder for a brake system. The master cylinder includes a cylinder body that has a bore therein; at least one piston provided in the bore to be moved forwards and rearwards; a liquid pressure chamber compressed by the piston; a brake liquid port that is connected to a reservoir and configured to supply brake liquid to the liquid pressure chamber; and a brake liquid discharge opening for discharging the brake liquid in the liquid pressure chamber to a wheel brake of a wheel. A valve unit that includes a check valve to control the unidirectional flow of the brake liquid that is introduced from the reservoir to the liquid chamber and a fluid resistance hole opened in two directions and is smaller than the size of the brake liquid discharge opening is provided in the brake liquid port.