To reduce manufacturing cost and guarantee sure tightening.

A jaw alloted to a prescribed tightening diameter area D1-D7 and provided with a tightening surface with teeth has teeth on the trunk surface of an insert 20. The insert 20 is formed symmetrically at both sides of a center line M, further on the center line M and arranged in the recessed part 15 opened against the tightening surface. The recessed part 15 is started from the end surface of the jaw and formed on the jaw nearly in a right angle against the end surface and meantime, the insert 20 is projected from the tightened surface by the teeth and surrounded by the wall of the recessed part 15 with a contact angle beyond 180°.