A fitting piece of a spring fitting body prevents a nut from being moved by mounting a spring slightly smaller than the contour of a bolt so as to be fitted into a screw groove arranged at a leg part of the bolt. An engagement part (2) engaged with a base end part of the spring is provided on the outer circumference of a rod-like core (1), and a collar (3) having a diameter slightly larger than the outer diameter of the spring is provided on the outer circumference of the rod-like core (1).
