In medical sensor networks, sensor nodes on a body of the patient sense the vital data and deliver them to the server system through mobile devices or gateways in the middle. However, when these linking devices do not function because of the battery outage or the system fault, the server system could not get patients' data during that period, and this situation could lead to serious problems. In this paper, to cope with this problem, we propose a distributed authentication mechanism in which substituted devices could be authenticated to deliver patients' data to the server system in the unusual situation.