A novel processing method was proposed to enlarge a partial diameter in the middle of a round bar with an experimental size of 10mm. in diameter. To confirm that the processing method is applicable also to shaft with a practical size of 25mm-60mm in diameter for general industrial machine, a large-scale processing machine was developed. Deformation behavior of diameter in a processed part during processing process was made to an expression. The several influences, such as bending angle theta, axial-compressive stress sigma(c), diameters D-0, rotation number N of shaft and curvature radius p, on the increase behavior of diameter were estimated quantitatively.