To provide a technique achieving an address conversion processing of a message without holding address correspondence information in preparation for an incoming call in an address converter, in a communication service network using an SIP (Session Initiation Protocol).

An address converter is installed in a boundary part between an access network and a core network and is used. The address converter is configured to receive from a user terminal a registration message including the address of the user terminal as information of a contact header, to determine an external address being a core network side address in the address converter for the user terminal, to add the external address to the registration message as information of a prescribed header, and to transmit to a SIP server the registration message with the external address added thereto by including the registration message in a packet in which the external address is designated as a transmission source address.

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