Our body consists of many body parts that are compliantly connected with each other by muscles and ligaments, and their behavior emerges out of the synergy of the whole-body dynamics. Such synergistic behavior generation is supposed to contribute to human adaptive movement such as walking. This paper describes designing synergistic walking of a whole-body humanoid robot whose joints are driven by artificial pneumatic muscles antagonistically. We propose to take an incremental design approach to deal with the complicated dynamics of the system. As a result, we can determine control parameters that govern whole-body behavior. We experimentally demonstrate that the humanoid walks stably with a simple limit-cycle controller.