This invention relates to provide a communication network system, a gateway, and a data communication method, in which the gateway has an advanced functionality. A person who issues an access request can retrieve a desired access destination easily by the following configuration: functions of routing information providing and authentication processing are added to a gateway that performs protocol conversion between two different communication networks; when an access request is issued from a public communication network such as Internet to a terminal in a local (private) communication network connected to the gateway, a request terminal is authenticated to enable prevention of unauthorized data writing and reading; and an access request user who succeeded in the authentication is provided with an active terminal list comprising accessible terminal information, or with a user condition table.