PURPOSE: To facilitate a coupling securing work by a method wherein a staged sleeve is engaged with the staged hole of a power piston, and the large part of the outer side of the staged sleeve is coupled and secured to a power piston by means of a securing means, e.g. a lock nut, caulking.

CONSTITUTION: A large part 2a of a power piston 2 having a master cylinder to the piston of which an output shaft 4 at the tip of a small part 2b is coupled is engaged internally of a large hole 1a formed in a housing 1. The right end opening part of the large hole 1a is sealed with a plug 11. A power chamber 13 is partitioned between a plug 11 and a power piston 2, a staged hole with a bottom is formed in the right end shaft part of the power piston 2, and a small part 21a and a large part 21b of a staged sleeve 21 are engaged with a small hole 2d and a large hole 2c, respectively. Further, a stopper ring 22 and a sleeve 23 are engaged internally of the large hole 2c, and the sleeve 23 is secured in a way that the sleeve is joined in a screwed-in state with the large hole 2c of a lock nut 24 secured to the power piston 2 by caulking.