To surely prevent looseness by singly using a hexagon socket bolt.

The shape of a conventional hexagon socket bolt is changed and divided into two parts of a cylindrical cap with a conical hole and a hexagon socket conical head male thread fitted thereto, and the cylindrical cap with a conical hole is formed into a non-parallel shape by slightly making the bottom surface of the cylindrical cap with the conical hole inclined to an axially right-angled plane and setting the height of the top surface vertically extended from the bottom end of the inclined bottom surface lower than a parallel plane to the inclined bottom surface, whereby a bolt composed of a set of two parts and configured so that both parts are suitable for each other is constituted. The inclined bottom surface of the bottom surface-inclined cylindrical cap with a conical hole closely contacts with a fastened member with an axial line slightly inclined, and the looseness of the bolt can be prevented by a physical effect by the slight inclination of an axial line of a hexagon socket bolt to be fitted thereto and a wedge effect by both tapered parts.