Many hemiplegia patients use the ankle-foot orthosis (AFO) to prevent foot-drop when they walk. However, it is difficult to walk smoothly because conventional AFOs have high rigidity. In order to support natural gait of hemiplegias, in this study, a technique to regenerate energy is applied, and a new self-powered semi-active AFO combining a DC motor and a step-up chopper circuit is developed. In this method, it is possible to drive a long time safely and the damping on an ankle joint can be controlled. From gait experiments, this study show that developed AFO can be rotated the ankle joint smoothly, be charged battery by regenerating energy loss during a gait, and prevent foot-drop. Hence developed AFO can be expected to have high gait improvement effect than the conventional type. Furthermore, developed AFO shows high electricity recovery (86.5 %).