Abstract: After a bolt (3) and a double-structured nut (6) are fastened to each other, it is not possible to unfasten the bolt (3) and nut (6), because a predetermined part of the nut (6) is designed to break when subjected to a predetermined torque necessary for fastening the nut (6), and thereby permitting the remaining part of the nut (6) to remain fastened securely to the bolt (3) after the fastening operation of the bolt (3) and nut (6) is performed. The nut (6) double-structure including a cylindrical cover member (7) may alternatively be utilized for a similar double-structured bolt head (42) having a protective cover (43).