a cylinder body in a cylindrical form having one end closed and including a discharge passage and a supply passage for a brake fluid, the supply passage being communicated with a reservoir;

a piston slidably disposed in the cylinder body, the piston and the cylinder body defining a pressure chamber therebetween for supplying a fluid pressure to the discharge passage; and

a piston seal provided in a circumferential groove formed in the cylinder body, the piston seal having an inner circumferential surface thereof in slidable contact with the piston and being capable of sealing the pressure chamber from the supply passage;

the piston seal comprising: an annular base portion; an inner circumferential lip portion projecting from an inner circumferential side of the base portion and being in slidable contact with an outer circumferential surface of the piston; an outer circumferential lip portion projecting from an outer circumferential side of the base portion and being in abutment with the circumferential groove of the cylinder body; and an intermediate projecting portion being formed between the inner circumferential lip portion and the outer circumferential lip portion of the base portion and projecting beyond the outer circumferential lip portion, the intermediate projecting portion including a cutout groove formed on a tip side thereof,

wherein, when the piston moves back, brake fluid in the supply passage is supplied to the pressure chamber through a gap between the outer circumferential lip portion of the piston seal and a bottom surface of the circumferential groove, and the cutout groove of the intermediate projecting portion, and

wherein a cutout groove is formed on a tip side of the outer circumferential lip portion.