To provide a coil spring type locking element and a fastening structure that both require fewer or no turns for their attachment or detachment and show least looseness even when loaded considerably.

This coil spring type locking element comprises a band plate spiraling about a center shaft at a predetermined inclination and at gradually increased radii of curvature and having an externally threaded taper face 11 on its external surface. The locking element is preferably formed at its one end into a flat surface and at its internal surface into a cylindrical surface to fasten bearings or gears to the shaft. The fastening structure comprises the above locking element, a shaft 2 having a groove 22 and a left side wall or fastening face 23 juxtaposed, and a clamp nut 4 having an internally threaded taper face 41. As the nut 4 is fastened to lock the locking element in the groove 22, a fastened object 3 is set in a place between the locking element and the left side wall or fastening face 23.