A piston connected with a piston rod is fitted in an outer tube having a hydraulic fluid sealed therein, with the piston rod projecting out of the outer tube. A seal part for the piston rod includes a rod guide, a seal case, an oil seal, a dust seal and a securing member, which are inserted into an end portion of the outer tube and secured by first and second staked portions. The sliding part of the piston rod is sealed with the rod seal and a seal lip of the oil seal. The area between the seal case and the outer tube is sealed with an O-ring and a seal member. The pressure resistance of the seal part can be increased to a considerable extent by two-point staking by the first and second staked portions and two-stage sealing by the rod seal and the oil seal.