To improve safety by eliminating previous setting of a public key, management of public key/secret key and a key registration center.

A prime generation part 102 generates a random number R in a random number generation part 105 based on session information (t) with respect to transmission side identification information IDa and reception side identification information IDb and generates a large prime. A session key generation part 103 generates a cryptographic key K1 as a session key Ke based on the prime and file information F. A master key generation part 106 ciphers the plain sentence M110 of a file by the cryptographic key K1, generates a cipher sentence C112, decentralizes the cryptographic key K1 and generates two cryptographic keys a109 and b108. A mail control part 104 adds a cryptographic key a 112 to the cipher sentence C112, transmits it to a user B as a mail main body, adds the cryptographic key b108 to the mail to which a session key Ke 107, session information (t) and file information F are added and sends the mail later as an auxiliary mail.