To improve security function so that the protection of a fastening part can be easily released by an operator who receives a due authority and not released by a visitor.
The relative movement of a cover inside tube to a cover outside tube is restricted between a restriction position where a ball release groove is not allowed to communicate with a ball receiving hole and a release position where the ball release groove is allowed to communicate with the ball receiving hole. When the cover outside tube is moved to the release position, a keeper receiving part projected from the base end face of the cover inside tube is formed on the cover outside tube. Balls pressed inward by the opening edge part of the ball release groove ride on and a reengaged with the ball receiving hole and a ball locking groove, and the cover inside tube is arrested by a holder. Balls pressed outward by the opening edge part of the ball locking groove ride on and are engaged with the ball receiving hole and the ball release groove, and the cover inside tube is released from the holder. A keeper member inserted between the cover outside tube and an object to be tightened is slightly returned and engaged with the keeper receiving part.