A braking device includes a guide piston which surrounds the outer periphery of a rod part of a second slave piston, a cup seal which is mounted on the second slave piston, and a seal member which is provided on the guide piston. The second slave piston includes an enlarged diameter portion which comes into contact with an axial-direction surface of the guide piston when the second slave piston is in the initial position, and the enlarged diameter portion is provided with a cut-out section which extends in the internal diameter direction of the second slave piston.