A master cylinder includes a cylinder housing having a cylinder bore, a seal cup disposed inside the cylinder bore, and a piston slidably disposed in the cylinder bore in sliding contact with the seal cup and defining a pressure chamber in cooperation with the housing and the seal cup. The piston includes a connecting passage which fluidly connects the pressure chamber with an atmospheric pressure chamber when the piston is in a retracted position. The seal cup cuts off fluid communication between the pressure chamber and the atmospheric pressure chamber through the connecting passage when the piston moves in its axial direction by a prescribed distance from its retracted position. The connecting passage is a stepped hole having a first end portion with a small diameter which communicates with the pressure chamber and a second end portion with a large diameter which opens onto an outer peripheral surface of the piston. The cross-sectional area of the first end portion is such as to produce a throttling effect.