Recently RFID system is used in various fields such as distribution industry, medical industry and military service. The technology for protecting individual privacy is necessary to adapt RFID system in several applications. This paper proposes an authentication protocol which conducts mutual authentication between back-end database and tag using hash function. The proposed protocol satisfies various RFID security requirements : mutual authentication, anonymity, confidentiality, integrity, replay attack, location trace. This protocol reduces the time for authentication minimizing the number of hash operation in back-end database.