It is possible to perform encrypted communication between members of a group while assuring a sufficient security compatible with a change of the members. An IC card having the same fixed code F(a) is distributed to all the staffs of company A. When communication is performed between a staff α and a staff β belonging to the same project group, α of the transmission side writes an arbitrary variable code V(1) in the IC card (100a) so as to generate encryption key information K(a1) according to F(a) and V(1) in the IC card and encrypt data D1 by using K(a1). β of the reception side writes the variable code V(1) received from α in the IC card (100b) so as to generate encryption key information K(a1). By using this K(a1), the encrypted data D2 received is decrypted to obtain data D3. The fixed code F(a) is different for each of the companies and rewrite-disabled so as to assure security.