To fix a base piston to a cylinder and to house a free piston by fixing both end parts of the base piston by means of a slip off preventive member and housing the free piston inside a straight pipe.

Both end parts of a base piston 14 are fixed by means of C-rings 58A, 58B engagingly fitted to the inside face of a straight pipe constituting a cylinder 11. Therefore, the base piston 14 can be fixed to the cylinder 11 when locking parts (ring type grooves 59A, 59B) for the C-rings are simply arranged on the inside face of the straight pipe. On the other hand, a free piston 15 is housed in the straight pipe of the cylinder 11 in which the piston 12 is housed and the base piston 14 is fixed. Therefore, the straight pipe which is already machined for securing slidability of the piston 12 is also used for sliding of the free piston 15, so that the free piston 15 can be housed in the cylinder 11 by means of a simple structure.