As a broadcast message authentication method in wireless sensor networks, μTESLA enables sensor nodes efficiently authenticate message from base station (BS). However, if we use μTESLA that has very short length of key slot in unattended wireless sensor network (UWSN), sensors may calculate a huge amount of hashs at once in order to verify the revealed secret key. In contrast, if we set the length of μTESLA's key slot too long in order to reduce the amount of hashs to calculate, BS should wait out the long slot time to release key. In this paper, we suggest variable key slot μTESLA in order to mitigate the problem. As showing experiment results, we prove that our suggestion improve sensor node's response time and decrease of number of hash function calculation.※ 본 연구는 인하대학교의 지원에 의하여 수행되었습니다.First