A master cylinder includes primary and secondary pistons both slidably mounted in the cylinder bore of a cylinder. A stopper plate is disposed in the cylinder bore behind the secondary piston. A groove having a larger diameter than the piston sliding surfaces is formed in the inner wall of the cylinder bore at its axially intermediate portion. The stopper plate includes a pair of diametrically opposed flanges having an outer diameter larger than the diameter of the piston sliding surfaces and smaller than the diameter of the groove. The flanges are received in the groove and engageable with a shoulder formed at the rear end of the groove, thereby preventing excessive backward movement of the secondary piston. The stopper plate has a dimension perpendicular to the direction in which the flanges extend and smaller than the diameter of the piston sliding surfaces. Thus, the stopper plate can be inserted into the cylinder bore with one of the flanges located ahead of the other.