When a location of a portable terminal connected to another network beyond a packet communication network side changes, the gateway apparatus that is arranged between a mobile circuit switching network and a packet communication network (IMS network) and that performs mutual conversion between a circuit switching protocol and a packet communication protocol to execute voice communication, exercises control such that at least one of a sequence number and a timestamp in a packet destined to a portable terminal and output to the packet communication network, is made to vary continuously before and after movement of the portable terminal, in case the portable terminal connected to a network beyond the packet communication network side, moves its location.