A screw is the machine element being used most closely, and it is being used for the various things, machine and furniture, etc. But, if a screw isn't fastened properly, looseness occurs by the vibration and the shock. It is necessary to control the axial force of screw as a countermeasure in this problem. Such a problem can be solved if the axial force of the small screw can be controlled. However, the research of small screw isn't being done very much. Because of that, the character of small screw isn't appreciated, and such a problem be happening now. This research tried elucidation to the basis character of small screw, to solve a problem about tightening of small screw.