To provide a screw fastening method that ensures a firmly locking function in a normal condition at a screw fastening portion and, when intentionally fastening or loosening a screw, does not hinder the fastening or loosening work.

The screw fastening method of the invention ensures the firmly locking effect by filling thermoplastic material in a fitting portion between a male screw part and a female screw part when fastening the screw. When fastening or loosening the screw, the thermoplastic material is melted by using a screwing tool which generates heat, thereby smoothly fastening or loosening the screw. Further, when executing the conventional screw locking method a plurality of times, the locking effect is seriously lowered so that the screw cannot be used repeatedly. On the contrary, in the screw fastening method of the invention, it is possible to repeatedly use the screw by applying heat and appropriately adding the thermoplastic material.