In the case of some physically weak persons such as elderly persons, the environment perception ability is sometimes deteriorated also. To assist the daily living motion of those people, power-assist robots with the perception-assist have been proposed. The power-assist robot with the perception-assist assists not only the user's motion but also the user's interaction with an environment, by applying the modification force to the user's motion if it is necessary. In the case of lower-limb motion, the walking is the very important motion for a person to achieve daily activities. Some elderly persons might stumble on the bump and fall down in the stairs because they might not be able to lift own lower-limb well and/or recognize stairs correctly. In this paper, specifically, the stairs-ascending/descending assist for a lower-limb power-assist robot is proposed to prevent the user from falling down in stairs. ZMP of the user is taken into account in the proposed method. The effectiveness of the proposed method has been evaluated by the performing experiments.