Patent ID: 7963113

Claim:
A cylinder device comprising: a cylinder having a annular seal groove which is formed in an inner circumference thereof and includes: a bottom surface extending in circumferential direction of the cylinder; a cylinder hole bottom portion side surface and a cylinder hole opening side surface; a piston movably inserted into the cylinder and a cup seal fitted into the seal groove for sealing between the cylinder and piston, wherein the cup seal comprises: a base portion having a base end face contacting with the cylinder hole opening side surface of the seal groove; an inner circumferential lip extended from the inner circumferential side of the base portion toward the bottom portion of the cylinder hole and having an inner circumferential surface slidingly contacting with the piston; an outer circumferential lip extended from the outer circumferential side of the base portion toward the bottom portion of the cylinder hole and having an outer circumferential surface contacting with the bottom surface of the seal groove and a plurality of elastic projections provided along the circumferential direction thereof and projecting from the outer circumferential side of a leading end of the inner circumferential lip toward the cylinder hole bottom portion and a length of the cup seal from a leading end portion of the elastic projection to the base end face of the base portion is set longer than an axial direction of the seal groove.