Most RFID (Radio Fequency IDentification) Tag security schemes assumed that the backward channel from tags to a reader is safe from eavesdropping. However, eavesdroppers near a tag can overhear message from the tag illegally. This may cause some privacy issues because the backward channel eavesdropping means the expose of personal information related to the tags that each person has. In this paper, we propose a method to protect the backward channel from eavesdropping by illegal readers. The proposed scheme can overcome the problems of conventional schemes based on tree-walking algorithm. It is shown that the proposed method can provide the probability of eavesdropping in some standardized RFID tag system such as EPCglobal, ISO, uCode near to '0'.