A cup seal 21 includes a base portion 21a disposed on a cylinder bore opening portion side of a seal groove, an inner circumferential lip portion 21b extended from an inner circumferential side of the base portion toward a cylinder bore bottom portion and having an inner circumferential surface slidably abutting against an outer circumferential surface of a piston (plunger), and an outer circumferential lip portion 21c extended from an outer circumferential side of the base portion toward the cylinder bore bottom portion and having an outer circumferential surface abutting against a seal groove bottom surface. An abutment surface 21f having a protruded end protruded toward the cylinder bore opening portion and abutting against a surface on the cylinder bore opening portion side is provided on a base end surface 21e of the base portion, and a contact pressure adjustment surface 21g which is brought into non-contact with an inner circumferential side of the opening-side-surface in a non-operational state is provided at a position closer to the inner circumference than the abutment surface.