The hollow cylindrical member 210 has the recessed portion 211 in its inner cylindrical surface, while the linearly moving member 212 has a cylindrical rod 213 and a pair of disk-like collars 214 and 215 which are secured to the rod 213 and are in contact with the inner cylindrical surface of the hollow cylindrical member 210 at their annular outer edges in such a manner as to be slidable in the B