Both a management server and a validation server are installed. Both a terminal and a terminal register setting information which is usable in an encrypted communication in the management server. When carrying out the encrypted communication, the management server searches the registered setting information for coincident setting information. The management server generates keys for the encrypted communications which can be used by the terminals, and delivers these generated keys in combination with the coincident setting information. The management server authenticates both the terminals in conjunction with the validation server. Since the terminals trust such results that the management server has authenticated the terminals respectively, these terminals need not authenticate the respective communication counter terminals.