A master cylinder that can sufficiently compensate for an ineffective fluid amount at an initial stage of a stroke, even at a time of sudden braking in a case where a hydraulic control unit is disposed between the master cylinder and a brake device, is provided. The master cylinder includes a control valve that releases hydraulic pressure in a large diameter pressurizing chamber to a reservoir side when the hydraulic pressure in the large diameter pressurizing chamber reaches a fluid-release pressure corresponding to hydraulic pressure introduced via a branched flow path that branches from a communication flow path that connects a hydraulic control unit and a small diameter hydraulic chamber. A restrictor is disposed in the branched flow path for introducing the hydraulic pressure to the control valve.