A plastic deformation method of a tightly wound coil spring includes a regulating process and a plastic deformation process. In the regulating process S10, a tightly wound coil spring 28 is inserted into a pipe 60, and deformation of the tightly wound coil spring in a tightly wound coil spring radial direction is regulated by the pipe 60. In the plastic deformation process S20, a compression force is applied to the tightly wound coil spring 28 inserted into the pipe 60 from one end or both ends in a tightly wound coil spring axial direction. Then, a rod 64 is inserted into the pipe 60 from a left-end opening portion of the pipe 60, and the compression force is applied to the tightly wound coil spring 28 by a pressing force of the rod 64.
