To decrease the numbers of ciphering keys and deciphering keys of respective terminals by transferring ciphered data to an opposite terminal and allowing the opposite terminal to decipher the ciphered data by using a deciphering key for communication with a server.

The respective terminals PC-A to PC-G transfer to a server 10 the data obtained by ciphering data and the name of opposite terminals to which the data should be transferred by a cipherer 2 by using cryptographic keys for communication with the server 10. The server 10 receives the ciphered data and deciphers the data by a decipherer 11 by using a deciphering key, and reciphers the deciphered data by the cipherer 2 by using the cryptographic keys of the opposite terminals to which the deciphered data should be transferred. Further, the reciphered data are transferred to the opposite terminals, which decipher the reciphered data by a decipherer 4 by using the deciphering key for the communication with the server 10. Consequently, the ciphering keys and deciphering keys of the terminals PC-A to PC-G are greatly decreased in number.