A threaded member has an axial direction. A thread is formed on the threaded member in the axial direction. At least one receiving recess is formed on a circumferential surface of the thread of the threaded member. At least one sensor member is disposed in the receiving recess for detecting tension applied to the threaded member in the axial direction. When a tightened article is tightened by the threaded member, an extension force/tension is applied to the threaded member in the axial direction. The sensor member is positioned in adjacency to the thread of the threaded member so that the sensor member can precisely detect the extension extent of the threaded member for precisely obtaining the tightening force applied to the threaded member.