To provide a non-contact IC tag data encoding/decoding method, its system, and a non-contact IC tag for preventing leakage of tag data written and stored in the non-contact tag.

This non-contact IC tag has a unique ID stored in a storage mean during its production, while a computer device is provided with an input means, a means of reading the ID from the non-contact IC tag via a reader/writer, an encoding means for encrypting the data input by the input means by using the ID, a means of writing the encrypted data in the storage means of the non-contact IC tag via the reader/writer, a means of reading the data stored in the storage means of the non-contact IC tag into the computer device via the reader/writer, a decoding means for decoding the data read from the non-contact IC tag by using the data and the unique ID, and a means of outputting the data decoded by the decoding means.