To rationally and safely perform a commercial transaction by using a virtual card on the communication line of internet or the like and performing communication after converting processing such as enciphering is temporarily executed to card information.

In the case of registration from a user 12 to a certificate agency CA 13 and a card company PG 14, first of all, the user 12 generates a disclosure key and a cryptographic key, registers this disclosure key to the CA 13 and gets the certificate of that disclosure key. Besides, when the user 12 applies the registration to the PG 14 together with the disclosure key, the PG 14 confirms whether the disclosure key of user 12 is registered in the CA 13 or not. Then, the CA 13 issues the certificate of the disclosure key registered by the user 12 to the PG 14. Similarly to a credit card, the PG 14 investigates identity and in the case of OK, the PG 14 sends a credit card number to the user 12. Thus, the transaction is simplified in comparison with a conventional commercial transaction using credit cards and without requiring any special equipment, the commercial transaction is easily performed.