An address translator carries out address translation between a private address and a global address on a source address or a destination address included in the header of a received packet. The translator includes an address translation processor for applying address translation information to the header area of the received packet on a network layer level and carrying out address translation on the network layer level by means of the address translation information. Thus, the translator can establish high transparency, high possibility to establish interconnection, impartial assurance for users, high throughput and high availability, and thus provide open measures minimizing interrupt actions of any communication as well as possible.