Abstract: An inner cylinder 6 is disposed in an outer cylinder 5, and the interior of the inner cylinder 6 is defined into an upper chamber a and a lower chamber b by a piston 3. A space between the outer cylinder 5 and the inner cylinder 6 defines an oil passage c which is communicated with the upper chamber a. A through hole-like plug which is attached to means P is formed in a bottom 5a of the outer cylinder 5. A female plug 11 and a male plug 12 are inserted from opposite ends of the plug accommodating means P, and a female screw portion 11n and a male screw portion 12b are formed on tip ends of the female plug 11 and the male plug 12, respectively, are threaded to each other. A recess 11d of the female screw 11n is brought into communication with the oil passage c through a first communication passage R.sub.1, and a recess 12d of the male screw 12n is brought into communication with the lower chamber b through a second communication passage R.sub.2.