Ankle-foot orthosis with a pneumatic rubber actuator, which is intended for the assistance and the enhancement of ankle muscular activities was developed. In this study, appropriate amount of power pattern of the device and the effectiveness of the system was investigated. The subjects were instructed to perform gait motion on the treadmill. The subjects were performed the gait motion wearing and not wearing the ankle-foot orthosis. We made a comparison the muscular activities between wearing and not wearing the ankle-foot orthosis. For the analysis of muscular activities, surface Electromyography (EMG) of the subjects were measured during gait motion. A Biodex system 3 (Biodex medical system Inc., Shirley, NY, USA) was used to measure the concentric iosokinetic strength of the plantar flexors of wearing and not wearing the orthosis on right side. The experimental results in gait motion showed that the muscular activities wearing the ankle-foot orthosis were reduced because of the assistance of ankle-foot orthosis and the concentric isokinetic strength values wearing it were increased because of the assistance of the pneumatic power. Therefore, we confirmed the effectiveness of the assistance in the developed ankle-foot orthosis.