Aiming at opening up a new stage of humanoid robotics, we have been studying on the mechanically soft structure for humanoids and have developed a full-body muscle-driven flexible-spine humanoid robots. In this paper, we discuss body structure of humanoids, describing the advantages of musculoskeletal flexible body and briefly introducing the previous works on constructing hardware and software of flexible spine robots. This paper also presents the design and development of a novel musculoskeletal humanoid named Kotaro