The pressure vessel for a reactor is composed of a body (straight shell) and a panel. Recently, the diameter of shells has increased because there is a tendency for pressure vessels to be designed on increasingly large scales. As a result, a tapered spacer is inserted between the shell and the panel. However, this increases the cost because of the need for welding. Thus, a solid shell, which is composed of a tapered spacer and a straight shell, is required. In this study, a new forging method for the fabrication of a solid shell was developed on the basis of a model experiment, and the deformation behavior was formulated from the result of the model experiment evaluating the forging process.