For the person having functional disorder in their legs, we have developed Hybrid Assistive Leg(HAL). The purpose of this study is to propose the method to control HAL. In this method, according to human walking, the walking task is divided into phases. Keeping COG be controlled through all phases by manipulating ZMP, it would be possible to generate the stable motions without falling down which have several speeds and step lengths. Using computer simulation, we would confirm the effectiveness of this method and the ability for applying this method to the rehabilitation program for handicapped people.