Animals exhibit astoundingly adaptive and supple locomotion under real world constraints. In order to endow robots with similar capabilities, we must implement large degrees of freedom, equivalent to animals, into the robots' bodies. For taming large degrees of freedom, the concept of autonomous decentralized control plays a pivotal role. However, a systematic way of designing such autonomous decentralized control system is still missing. Aiming at understanding the principles that underlie animals' locomotion, in our early studies, we focused on true slime mold, a primitive living organism, and extracted a decentralized control scheme. In order to validate this control scheme, this paper presents a soft-bodied amoeboid robot inspired by true slime mold. Significant features of this robot are twofold: (1) the robot has truly soft and deformable body stemming from real-time tunable springs and a balloon, the former is used for an outer skin of the body and the latter serves as protoplasm; and (2) a fully decentralized control using coupled oscillators with completely local sensory feedback mechanism is realized by exploiting the long-distance physical interaction between the body parts stemming from both the softness of the body and the law of conservation of protoplasmic mass. Experimental results show that this robot exhibits truly supple locomotion without relying on any hierarchical structure. The results obtained are expected to shed new light on design scheme for autonomous decentralized control system.