In order to realize an assist of independent life for people in need of care and relieve a physical burden for care worker, an active support splint driven by pneumatic soft actuator (ASSIST) to support a human arm has been developed. ASSIST is constructed with the appliance and the rotary-type soft actuator. Since a weight and hardness of device involve a danger of causing a serious accident for users or neighbors, this device is constructed with components which are a lightweight and have a mechanical softness. In this paper, the structure of developed device is described, and then the control method based on the center of pressure is proposed. Finally, the assist effectiveness using the proposed control method is experimentally discussed.