To provide a technology of a closing cap fixing device for stably and surely closing a bolt insertion hole arranged in a housing of a brake actuator.

A closing cap integrally forms a projection part of projecting a cap pin outside on an under surface of a base body part. A worker rotates the cap pin of the closing cap while being brought into pressure contact with a surface of an inclined face part 18b in an under surface 18a of a substantially central part 18A of the housing 18 when rotating the closing cap in the right direction by fitting the cap pin of the closing cap in a cap pin fitting hole 19b. When further rotating in the right direction, the cap pin runs on the apex 18e of the inclined face part 18b. The worker feels a click feeling in hand. When further advancing rotation in the right direction, the cap pin stops by butting against a stopping projection part 18d by advancing while being brought into pressure contact with an surface of a plane part 18c, and the worker confirms a stopping position of the cap pin by the touch of the hand.