A plastic deformation method of a tightly wound coil spring includes a regulating process and a plastic deformation process. In the regulating process, a tightly wound coil spring is inserted into a pipe, and deformation of the tightly wound coil spring in a tightly wound coil spring radial direction is regulated by the pipe. In the plastic deformation process, a compression force is applied to the tightly wound coil spring inserted into the pipe from one end or both ends in a tightly wound coil spring axial direction. Then, a rod is inserted into the pipe from a left-end opening portion of the pipe, and the compression force is applied to the tightly wound coil spring by a pressing force of the rod.
