To provide an audio gateway unit for easily introducing IP telephone service in an environment where a private telephone exchange is used.

A communication system comprising the audio gateway unit 30 is nearly made up of a telephone terminal 10, the private telephone exchange 20, the audio gateway unit 30, a call agent 40, an IP network 7, and a public telephone network 8. The audio gateway unit 30 connects office lines 9 from the private telephone exchange 20, subscriber lines 4, and an xDSL modem 1 for transmitting/receiving a digital signal over the subscriber lines 4 together; carries out mutual conversion between an audio band signal and IP packets; and has a function for transmitting/receiving the audio band signal and the IP packets.