In a gateway apparatus for interconnecting different types of communication networks that are a line network and a packet network, a method and an apparatus for eliminating sound interruptions that would otherwise occur due to
delay or loss of sound encoded data, minimizing the degradation of
sound quality, and maintaining a short
delay for
telephone communication. A sound
data processing circuit (550) of the gateway apparatus compares an expected value of the number of sound encoded data as expected outputs to be developed from a multiplexed-data separating circuit (200) in a unit period with the actual number of sound encoded data. If the actual number of sound encoded data is below the expected value, then the sound
data processing circuit (550) generates encoded data for causing a destination terminal to execute an error encapsulation, and packetizes and transmits the generated encoded data together with the sound encoded data to the packet network from a transmission circuit (801). If acquiring no sound data from a reception circuit of the packet network in a given period, the sound
data processing circuit (550) generates a
signal indicative of no acquisition, selects either generation or disposal of the encoded data, and sends the
signal to the line network from a data
multiplexing circuit (900) and a line network terminating circuit (100).