To provide a bolt to be required for a low coupling strength like a case that when a load higher than a given value is inputted, an impact applied on a human body is relaxed through disconnection of a nut from the bolt.

A brittle means 3 to release joining in a screwed-in state with a bolt due to easy deformation of a nut body is arranged at a part of a nut body 2. The brittle means 3 is formed such that the part in less than an approximate 180° arc in a peripheral direction of the nut body is notched and the plane of the nut body 2 is formed in a C-shape. Thereby, when a bolt is inclined, the nut body 2 is wrenched open at the notched part, the bolt is pulled out therefrom and joining in a screwed-in state between the nut and the bolt is released.