To facilitate locking and unlocking work even though in simple locking structure and to prevent a locking part to lock from projecting outside of a large diametrical pipe.

The large diametrical pipe 1 and a small diametrical pipe 2 are locked by locking an eccentric rotor 6 against the large diametrical pipe 1 by rotating the eccentric rotor 6 against the small diametrical pipe 2 by contact of a contact body 8 with an inner surface 1A of the large diametrical pipe 1 at the time of relatively making the contact body 8 contact with the inner surface 1A of the large diametrical pipe 1 by mounting the contact body 8 on the eccentric rotor 6 by providing the eccentric rotor 6 free to rotate against the small diametrical pipe 2 on an eccentric position X6 eccentric against a shaft center X3 of the small diametrical pipe 2.