A master cylinder includes a cylinder, a piston slidably provided in the cylinder, an annular cup seal held on an inner circumferential surface of the cylinder, and a relief port formed in the piston. The piston and the cylinder define a pressure chamber for generating fluid pressure in front of the piston, and the cylinder and an outer circumferential surface of the piston define a fluid supply chamber which communicates with a reservoir. The cup seal is adapted to slide against the outer circumferential surface of the piston to separate the fluid supply chamber from the pressure chamber. The relief port provides communication between the pressure chamber and the fluid supply chamber. A control taper surface is formed on the outer circumferential surface of the piston rearwardly from the relief port. The control taper surface is tapered toward the front of the piston and is capable of abutting against an inner circumferential surface of the cup seal.