This paper describes a lower-limb power-assist robot which assists the perception of the user using laser range finders. If the robot has not found any problems in the user's motion, then the ordinal power-assist is carried out by the robot. On the other hand, if the robot has found some problems in the user's motion in an environment, the robot tries to modify the user's motion to walk properly. However, the user might fall by the perception assist because the perception assist generates the additional force without user's attempt. Therefore, in the proposed perception assist method, the robot prevents the user from falling by considering ZMP. The effectiveness of the proposed method was evaluated by the experiments.