As the importance of sensor networks for the realization of ubiquitous world is recognized, the various aspects research for static sensor networks, that does not support the mobility of node, is underway. Medical sensor network (MSN) assuming the mobility of node takes notice as one of the highlight USN applications, but the research on the security issue for the environments is not enough even though the issue is the most important point for the successful deployment. Thus, in this paper we propose an adaptive authentication mechanism taking consideration in the tradeoff between processing speed and the degree of security strength. Using threshold secret sharing, our authentication gives the proper reputation to sensor nodes according to their behavior pattern. Thus if a sensor node has the best reputation and its sensing information is urgent, then the sink node processes the sensing information at first and performs post-authentication. Our analytical results show that our authentication provides the efficient to MSN even in the severe node traffic.