Abstract: As a method of authenticating an RFID tag in order to reduce a load of a server and improve security, an RFID reader connects an RFID DB server through a network and communicate with a plurality of tags, requests tag information to the tag, and receives an identifier of an array having an index, an index of the array having the index, and an encrypted tag ID from the tag. The array having the index is created by using a master key corresponding to the identifier of the array having the index, which is received from the RFID DB server, an encryption key is created by extracting an array value corresponding to the index, and an tag ID is acquired by decrypting the encrypted tag ID by using the created encryption key.