To maintain the expanded/shrunk state of a fixed ring 10 and prevent the breakage of the ring 10.

Projections 26 are inserted into holes 16 at both ends of a fixed ring 10, and a load adjusting rod 30 is arranged between grip handles 21 and 22 as shown by solid lines. When a nut 34 is screwed, both handles 21 and 22 are narrowed, and the fixed ring 10 is expanded. The nut 34 is smoothly screwed by a lever action at this time. The nut 34 abuts on a cap 32 at the optimum expanded degree of the ring 10, and the expansion is maintained by its spring force. Since the ring 10 is expanded by the spring force, large expanding force is not abruptly applied to the ring 10, the expanding force is regulated, and the ring 10 is hardly broken. Lock points between the adjusting rod 30 and the handles 21 and 22 are formed into circular arc faces 27a and 28a, and the rod 30 can be smoothly opened or closed.