A gateway apparatus that interconnects a mobile circuit switched network and a mobile IMS network to implement the video phone includes a conversion unit. The conversion unit demultiplexes a multiplexed bit stream signal output from a mobile terminal connected to the mobile circuit switched network to take out one or more of a control signal, a compression-encoded speech signal, and a compression-encoded video signal. The conversion unit converts the control signal into one or more of a MEGACO signal, a SIP signal and a SDP signal, while converting the speech signal and the video signal, if necessary. The conversion unit also packetizes the speech signal and the video signal to transmit resulting packets to the IMS network.