The basic investigation was conducted for the diameter enlarging machine of a shaft by repeating cyclic torsion newly developed as a novel idea. With a concept repeating the alternate cyclic stress on a shaft subjected to axial-compressive force, the plastic working method to cause diameter enlarging deformation on a part of shaft was proposed. In this investigation, the diameter enlarging experiment and the model analysis by FEM with notice of the deformation behaviors on diameter enlarging process were conducted as axial-compressive stress σc and torsion angle θ for mild steel pipes of SGD400. About 300cycles of cyclic torsion was repeated the cyclic torsion angle θ=3°as a standard process condition to a shaft subjected to axial-compressive stress of σC/σy0 =1.0. In this condition, the shaft has enlarged to about 1.3 times of initial diameter (D/D0=1.3). The experimental value was in excellent agreement with the analysis value. Also, not only diameter enlarging deformation behaviors but also influence of machining conditions on the behaviors was clarified.