To provide a bolt in which an appropriate axial force can be easily visualized.

A transparent material 2 is inserted in the inside of a bolt body 1 by being passed through an axial direction, and a part in which a stain 3 is filled, a thin film 4 equivalent to the cover of an enclosing opening, and a stain sump 5 being a cavity connected to the thin film 4, are formed in the inside of the bolt body 1. When axial force is produced in a bolt during a fastening process, compressive force from a radial direction is applied to the enclosing part of the stain 3 by the relation of Poisson's ratio. When the bolt axial force reaches just a previously set appropriate value, the enclosing part is set to be broken by the magnitude of the compressive force from the radial direction applied to the enclosing part of the stain 3.