A braking device comprising: a guide piston (103) which surrounds the outer peripheral surface of a rod part (89a) of a second slave piston (88a); a cup seal which is mounted on the second slave piston (88a); and a seal member (103b) which is provided on the guide piston (103). The second slave piston (88a) comprises an enlarged diameter section (117) which comes into contact with the axial-direction surface (119) of the guide piston (103) when the second slave piston (88a) is in the initial position, and the enlarged diameter section (117) is provided with a cut-out section (121) which extends in the internal diameter direction of the second slave piston (88a).
