This invention is directed to prevention of degradation in the audio quality between a mobile terminal connected to a mobile high-speed network and a mobile terminal connected to a mobile line exchange network. A gateway apparatus connects a packet forwarding apparatus situated on a mobile high-speed network to a radio base station control apparatus situated on a mobile line exchange network via an IMS network, and further, when having received, from the packet forwarding apparatus, a request signal for changing the audio bit rate, converts the bit rate of an audio compression-encoded bitstream, which is stored in a packet received from the packet forwarding apparatus or from the radio base station control apparatus, to a bit rate requested by the request signal. The gateway apparatus then transmits the audio compression-encoded bitstream having the bit rate as converted to the radio base station control apparatus or the packet forwarding apparatus.