The development of assistive technologies has an important impact on user's activities of daily living and social participation. However, assistive technology identifies persons with disabilities. In this paper, the psychological effects of using a mobility aid were considered before forming the concept of a new device. To identify the detrimental factors, a qualitative research approach was adopted in a study on 12 older persons. The investigation revealed that older persons had a dilemma in using mobility aids between maintaining body functions and extending mobility activities. In addition, there are two sides to the dilemma, namely the function and the appearance of the body. The process to reducing these dilemmas was clarified and a psychological model was constructed. The generation process of the psychological conflict obtained from qualitative data was revealed by modeling details and taking movement, body function and body appearance into consideration. Based on this model we propose and develop a new concept of mobility aid, "Tread-Walk, " is four-wheeled, permitting the user to walk naturally while a servo motor amplifies the normal walking speed. Preliminary evaluations suggest that this device is both safe and effective for use by middle aged and older adults.