A gateway apparatus, comprising: a control signal transmission and reception unit adapted to exchange with a first mobile terminal connected to a packet forwarding apparatus on a mobile high-speed network a control signal, when congestion is detected in a mobile high-speed network; and a conversion unit adapted to convert, when congestion is detected on the mobile high-speed network and the bit rate of voice transmitted or received by the first mobile terminal has been changed to a changed bit rate, a bit rate of a voice compressed-and-encoded bit stream stored in a packet supplied from the packet forwarding apparatus from the changed bit rate notified by a notification scheme into a bit rate transmittable and receivable by a second mobile terminal connected to a radio network controller on a mobile circuit-switched network, and to transmit the converted compressed-and-encoded bit stream to the radio network controller, wherein the gateway apparatus is adapted to connect voice communication between the packet forwarding apparatus and the radio network controller via an IMS (IP Multimedia Subsystem) network, the notification scheme is executed using Real-time Transport Control Protocol, RTCP, Application, APP, or Codec Mode Request, CMR, and the control signal transmission and reception unit is adapted to transmit and receive the control signal using Session Initiation Protocol/ Session Description Protocol, SIP/SDP, characterised in that the control signal is indicative of whether a change of at least one of the bit rates of voice transmitted and received by the first mobile terminal is possible, according to both the first mobile terminal and the gateway apparatus, the control signal is indicative of the notification scheme of the changed bit rate between the gateway apparatus and the first mobile terminal, and the changed bit rate is notified by the notification scheme by a signal which is different from the control signal and is transmitted after exchanging the control signal. a control signal transmission and reception unit adapted to exchange with a first mobile terminal connected to a packet forwarding apparatus on a mobile high-speed network a control signal, when congestion is detected in a mobile high-speed network; and a conversion unit adapted to convert, when congestion is detected on the mobile high-speed network and the bit rate of voice transmitted or received by the first mobile terminal has been changed to a changed bit rate, a bit rate of a voice compressed-and-encoded bit stream stored in a packet supplied from the packet forwarding apparatus from the changed bit rate notified by a notification scheme into a bit rate transmittable and receivable by a second mobile terminal connected to a radio network controller on a mobile circuit-switched network, and to transmit the converted compressed-and-encoded bit stream to the radio network controller, wherein the gateway apparatus is adapted to connect voice communication between the packet forwarding apparatus and the radio network controller via an IMS (IP Multimedia Subsystem) network, the notification scheme is executed using Real-time Transport Control Protocol, RTCP, Application, APP, or Codec Mode Request, CMR, and the control signal transmission and reception unit is adapted to transmit and receive the control signal using Session Initiation Protocol/ Session Description Protocol, SIP/SDP, characterised in that the control signal is indicative of whether a change of at least one of the bit rates of voice transmitted and received by the first mobile terminal is possible, according to both the first mobile terminal and the gateway apparatus, the control signal is indicative of the notification scheme of the changed bit rate between the gateway apparatus and the first mobile terminal, and the changed bit rate is notified by the notification scheme by a signal which is different from the control signal and is transmitted after exchanging the control signal.