Abstract: An annular inward turned flange and a plurality of ribs are integrally molded on an inner peripheral surface of an cylindrical portion formed at an upper portion of a reservoir for a master cylinder. The ribs extend from a lower surface of the flange and are disposed in a distributed manner in a circumferential direction of the flange. An annular flat surface area is formed on an upper surface of the flange, and a tip end of a working oil filler is placed into close contact with the annular flat surface area. Thus, when a working oil is filled under pressure into the master cylinder and its output hydraulic pressure circuit through the reservoir by the working oil filler, the urging force from the working oil filler can be reliably supported, and the deformation of the reservoir by a filling pressure cannot be produced.