A cup seal includes a base portion disposed on a cylinder bore opening portion side of a seal groove, an inner circumferential lip portion 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 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. The cup seal also includes an abutment surface and contact pressure adjustment surface.