Authentification data (R) comprising random numbers generated by a reader/writer (200) is given to an IC card (100), encrypted by utilizing a secret key (a), and returned as encryption data (C). This encryption data (C) is decoded by utilizing an open key (.beta.) to authentify the IC card. A set of past given authentification data (R) is stored in an authentification data storage section (130): if another set of given authentification data (R) is the same as the set of past data, an encryption operating section (150) is prevented from executing an illegal statistical analysis technique by repeatedly giving encrypting operation the same set of authentiification data (R).