A Radio-Frequency-Identification (RFID) tag is a small and cheap device which is combined in IC chip and an antenna for radio communications. It emits an ID in response to a query from a radio communication device called as a reader. For this reason, the RFID tag is used for management of goods and it is used as a substitute for a bar code. However, RFID system may infringe on a consumer's privacy because it has a strong tracing ability. Although ID of a RFID tag can be encrypted, it is possible to pursue an object by tracing specific information. Therefore, we discuss the privacy protection using universal re-encryption proposed by Golle, Jakobsson, Juels and Syverson. Since the system does not protect a modification of the information on RFID tags, it can be exploited by an attacker. Therefore we point out two attacks using modification of the information on RFID tags. Moreover, we offer two proposed schemes for addressing the problem.