Not only the motor function, but also the sensory function to recognize environments is often deteriorated in elderly persons. Due to the decline of the perception ability, they may encounter accidents such as stumbling. The concept of perception-assist has been proposed to avoid those accidents by automatically modifying their motion using an exoskeleton robot. In perception-assist, the lower-limb exoskeleton tries to modify user's motion automatically for avoiding accidents such as stumbling. The stumbling may be caused by insufficient ankle dorsiflexion as well as lack of lifting up a leg. Therefore, in addition to the motion assist for hip and knee joints, ankle joints also should be assisted to avoid stumbling. Consequently, 3DOF (hip, knee, and ankle joints) motion assist is preferred in each leg in the lower-limb exoskeleton robot, although the minimum degrees of freedom to modify the tip motion of the leg on the sagittal plane is 2DOF. Thus, the power-assist robot with perception-assist has redundant degree of freedom in the user's foot motion modification on the sagittal plane. In the power-assist robot with perception-assist, the redundant degree of freedom can be used to control the modifying motion by utilizing a null space. In this paper, the effective cost function of the null space is proposed for considering the movable range of each user and control of the ankle angle for the robot's energy efficiency. The effectiveness of the proposed cost function was evaluated by conducting experiments.