Booster

A booster includes an output shaft (36) a base (36A) of which comprises a rod member (36B), and a holder (43) having a bottom (43A), a stop (42) being formed separately from the holder, the holder having a reduced diameter portion (43B), in which an outer coiled spring (41A) and an inner coiled spring (41B) are disposed, a piston (40) being disposed rearward thereof, and an increased diameter portion (43D) in which the stop is disposed, and a reaction disc (35) being disposed rearward thereof.