Abstract: A secret key of a second apparatus is stored in a relay apparatus. A first apparatus specifies secret information used to identify a common key, generates encrypted secret information by encrypting the secret information by using a public key of the second apparatus, and transmits the encrypted secret information to the relay apparatus. Then, the relay apparatus decrypts the encrypted secret information by using the secret key of the second apparatus to extract the secret information. The relay apparatus transmits the encrypted secret information to the second apparatus. The second apparatus decrypts the encrypted secret information by using the secret key of the second apparatus to extract the secret information. Finished messages corresponding to communication log information and the secret information are exchanged between the first apparatus and the relay apparatus and between the second apparatus and the relay apparatus.