Patent ID: 8434304

Claim:
A cylinder apparatus comprising: a cylinder body comprising: a bottomed cylinder bore; and an annular seal groove provided in the cylinder bore; a cup seal fitted into the seal groove; and a piston slidably inserted into the cylinder bore via the cup seal, wherein the seal groove comprises: a seal groove bottom surface extending in a circumferential direction; a bottom-side-surface on a cylinder bore bottom portion side; a opening-side-surface on a cylinder bore opening portion side; and a seal groove opening opened to a piston side, the cup seal comprises: a base portion disposed on the cylinder bore opening portion side of the seal groove and having a base end surface opposing to the opening-side-surface of the seal groove; an inner circumferential lip portion extended from an inner circumferential side of the base portion toward the cylinder bore bottom portion and having an inner circumferential surface slidably abutting against an outer circumferential surface of the piston; 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 the seal groove bottom surface, and an abutting force maximum portion is provided on the base portion side in an inner circumferential surface of the cup seal at a position where abutting force against the piston becomes maximum within the inner circumferential surface of the cup seal in a state where the cup seal is fitted into the seal groove and the piston is inserted into the cylinder bore, wherein the abutting force maximum portion is provided on a cylindrical surface formed in parallel with at least one of an inner circumferential surface of the cylinder bore and the outer circumferential surface of the piston.