Since the Mobile IP is defined under the assumption that a mobile node roams between networks conforming to the same communications protocols, mobile communications between IPv4 and IPv6 are not possible. Further, a translation of the location registration messages also requires translating the format between different protocol layers. To solve this problem, a mobile proxy apparatus 2 is provided between a home network 1a and a foreign network 1b governed by different communications protocols. The mobile proxy apparatus 2 has a DNS-ALG function, a translator function and a Mobile IP function, and, by combining these functions, performs address translation and format translation on Mobile IP messages and user packets. The MN4 has Mobile IPv4 and Mobile IPv6 functions and executes communication suitable for the communications protocol governing the network to which it moves.