Patent ID: 8347623

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, an abutment portion is provided on a base end surface of the base portion of the cup seal, the abutment portion is protruded from the base end surface of the base portion of the cup seal toward the cylinder bore opening portion so that a protruded end thereof abuts with the opening-side-surface of the seal groove, a contact pressure adjustment portion is provided on the base end surface of the base portion of the cup seal at a position closer to an inner circumference than the abutment portion, the contact pressure adjustment portion does not contact with an inner circumferential side of the opening-side-surface in a state where the cylinder apparatus is in a non-operational state; and the contact pressure adjustment portion contacts with the inner circumferential side of the opening-side-surface in a state where the cylinder apparatus is in an operational state.