One of the objects of the present invention is to provide a communication system in which biometrics can be utilized without leaking to a third person so that a strict personal authentication can be conducted. The communication system includes, storing a correspondence table in a card, storing a reference password which is formed by converting a part of biometrics of an authorized user in the card by using the correspondence table, reading a part of biometrics of a user by the card, converting a part of the biometrics of the user into a password by the card using the correspondence table, and checking the password against the reference password by the card, wherein the card and the user are authenticated if a the password and the reference password match in the step of checking.