PURPOSE: To securely grasp unauthorized use and to magnetize the security of wide area network by providing a security server in common to the host computers of respective networks.

CONSTITUTION: The security server 16 is connected in common to the host computers 10a-10d of four networks A-D. For example, when the host computer 10a of the network A receives login from a user terminal 14d of the network D, the host computer 10a requests the security server 16 to investigate the login password and user ID of the specified user terminal. Then the security server 16 provides the password and user ID to all the host computers 10b-10d and requests them to investigate them. The host computers 10a-10d read their communication record and answer to the security server 16.