PROBLEM TO BE SOLVED: To provide a screw which can reduce damage delivered to an object to be screwed together when its rotation force is applied thereto, and is difficult to loosen.SOLUTION: An intramedullary nail lug screw 11 to be threaded into a bone tissue includes: a screw body 12 and a threaded part 15 having thread grooves 13 formed spirally in the circumference surface of the screw body 12, and screw threads 14 formed along the thread grooves 13. In the threaded part 15, the diameter of the screw thread 14 is reduced with distance from the intermediate portion in the axial direction of the threaded part 15 toward the front end and the base end. The diameter of a screw thread 14a in the intermediate portion in the axial direction is larger than that of a screw thread 14b on the front end side and a screw thread 14c on the base end side.【課題】回旋力が作用した際に螺合相手に与えるダメージが少なく、かつ緩みにくいスクリューを提供する。【解決手段】骨組織に螺合する髄内釘用ラグスクリュー11は、ねじ本体12と、このねじ本体12の周面に螺旋状に形成したねじ溝13、およびこのねじ溝13に沿って形成したねじ山14を有するねじ部15とを具備している。ねじ部15は、ねじ山14を、ねじ部15の軸方向中間部から先端部と基端部とに向かって漸次小径に形成して、軸方向中間部のねじ山14aを先端部側のねじ山14bおよび基端部側のねじ山14cより径大とする。【選択図】図1
