A circular shaped soft robot is a kind of jumping robots. This robot has jumping capability when its elastic outer shell is deformed. It was confirmed that the jump height is increased when the bottom side of the circular robot was deformed to flat. The deformed shape was referred as a dish shape, and some methods were proposed to deform a circular shell to dish shape. The elastic outer shell consisting of three rings is the one of structures capable of deforming to dish shape passively. However, in previous researches, the conditions of rings deforming to dish shape was not clarified theoretically, and the adjustment of the elastic outer shell was executed experimentally. In this study, we analyzed the deformed shape of elastic outer shell with three rings. Then, the relationship between the rigidity of each ring and the deformed shape was investigated. The combination factor of flexural rigidity and torsional rigidity of the rings were derived for the suitable bottom flatness of the rings.