This paper proposes a new method of locomotion control, named "Stiffness Distribution Control (SDC)", to realize various locomotion of a closed link robot with mechanical softness. SDC directly defines the reference stiffness coefficient at each hinge by SDC string which represents the distribution of the stiffness. This simple method does not need costly calculation at all and thus is suitable for controlling closed link robots. We build "BIYOn" as the prototype of closed link robots, the variable stiffness hinge (VSH) of which is newly designed. SDC is experimented on BIYOn and then it's found that SDC is effective in a controlling locomotion in practice. We also develop a systematic optimization method of stiffness distribution composed of two-stage approach and succeed in optimization to obtain a fast and smooth rolling motion and a sudden stopping motion.