At the time of exchanging routing information of a remote site, it is examined whether there is duplicated routing information, and when the duplicated routing information exists, an address conversion rule is generated. With this, when performing communication, address conversion is performed only for the communication to a site having a duplicated network address. In case of communication to a site having no duplicated network address, by performing IP communication intact, it becomes possible to omit unnecessary address conversion processing in a gateway. Also, when there is a duplicated address, by assigning a new network address which is not used in a site under a gateway to the duplicated address included in the routing information, and distributing the newly assigned address, it becomes possible to transmit a packet to a gateway appropriate to each destination, when there are a plurality of gateways in a subnet.