PURPOSE: To decrease weight of a through hole part and increase its rigidity, by partitioning the extension through hole part of a tie rod in a thin plate disc part of a piston with a short cylindrical part, fitting a protector ring to the short cylindrical part, enveloping the tie rod with extensible boots and fitting one end of the extensible boots to said protector ring.

CONSTITUTION: A booster piston 2 is constituted by a center boss part 2a and thin thickness steel plate disc part 2b, adapted to a flange 3 protrusively provided integrally to the periphery of the boss part. A through hole 32 of the disc part 2b, extended through with a tie rod 11, is partitioned by a stepped short cylindrical part 49, integrally protruded to a front side of the disc part 2b. A synthetic resin protector ring 43 of external diameter almost equal to a large contoured part 49b is fitted to a small contoured part 49a of the stepped short cylindrical part 49. The protector ring 43 is formed such that its front surface covers a point end of the small contoured part 49a of the short cylindrical part 49. An extensible boot 33, surrounding the tie rod 11, is tightly secured with its front end part 33a to the peripheral surface of the tie rod 11, and its rear end part 33b to the small contoured part 49a of the short cylindrical part 49 and the protector ring 43 by an annular mounting groove 52.