Authentication frameworks currently available in wireless networks, such as those in the IEEE802.15.4 security standard, are intended for applications in versatile sensor networks; however, both health care staff and medical professionals find it difficult to set them up. In addition, it is expected that these frameworks will increase computational burdens when applied to small vital sensors. It is therefore necessary to devise security arrangements that are easy to setup and consume less power for body area networks (BANs) consisting of vital sensors. In this paper, we evaluate our proposed zero-admin authentication system with reduced computational burden on a real vital sensor that has limited RAM/ROM and CPU power with a light-weight battery. Our proposed system expects a re-keying mechanism using vital data suitable for small sensors and uses one-way wireless communication for re-keying. The keys are kept their strength by infrequent updates of users' conditions. We discuss the protocol and then evaluate the re-keying mechanism with a typical cryptography on the real sensor device. Our evaluation shows the effectiveness of the proposed authentication system for practical use and raises additional issues that must be resolved for improved performance.