The ultimate role of assistive robots is not only supporting user's physical functions but also improving the quality of user's life. Designing such robots is not so easy, because most of the developers do not know about the real "effect" of assistive robots on the users' lives. We propose a concept design method of assistive robots which focuses on the effects of robots on the entire life of human. We introduce the concept of ICF (International Classification of Functioning, Disability and Health) in order to model the interactions among many factors in human activities. The developer uses the models as design knowledge, and defines the requirements along the use scenarios of the assistive robots. We utilize SysML (Systems Modeling Language) in writing the knowledge and scenarios. We developed a prototype of design support software which implemented the design method. In this paper, we describe the concept and the details of our method using some case examples of a mobility assistance robot, and present the future direction.