Polychaete worms move effectively using pedundulatory locomotion (i.e., body undulation coordinated with parapodia motions) and peristalsis in response to the environment. However, the essential control mechanism underlying adaptive multi-modal locomotion in polychaetes remains unclear. In this study, we observed the locomotion when a part of the body was placed in a narrow space and found that the worm exhibited a mixed locomotor pattern along the body. Based on this finding, we proposed a decentralized control model that can produce the adaptive switch between the pedundulatory and peristaltic locomotion in polychaetes and validated it in simulation.