Electronic tag data writing method and electronic tag read/write apparatus

In an electronic tag data read/write apparatus, an electronic tag (30) receives a response request from a reader/writer (10), generates a random number in a random number generator (34), stores the random number in a memory (33), and transmits the generated random number to the reader/writer (10). The reader/writer (10) conducts masking processing on write data on the basis of the random number per bit length of the received random number, repeats the masking processing until all of the write data are subjected to masking processing, and then transmits the masked write data to the electronic tag (30) collectively. The electronic tag (30) conducts deciphering processing on the received masked write data per bit length of the random number stored in the memory (33), repeats the deciphering processing until all of the masked write data are deciphered, and writes the write data into the memory (33).