A gateway apparatus transmits and receives control signal indicative of whether bit rates of voice transmitted and received by mobile terminal connected to packet forwarding apparatus on mobile high-speed network can be changed when congestion is detected in mobile high-speed network and of notification scheme of changed bit rate. When congestion is detected on mobile high-speed network and bit rate of voice transmitted or received by the mobile terminal has been changed, the gateway apparatus converts voice compressed-and-encoded bit stream stored in packet supplied from packet forwarding apparatus from the changed bit rate into bit rate transmittable and receivable by a mobile terminal connected to radio network controller on mobile circuit-switched network and transmits the converted compressed-and-encoded bit stream to radio network controller.