A plurality of partial networks can work independently from each other, and each partial network connects together several nodes including a connection node for transmission of audio signals. A connection network connects respective connection nodes of the respective partial networks so as to connect the plurality of the partial networks with each other through the connection nodes. The connection node of the partial network operates when the audio signal is transmitted from the partial network to the connection network, for forwarding the audio signal from a transmission channel used for carrying the audio signal in the partial network to another transmission channel to be used for carrying the audio signal in the connection network, and operates when the audio signal is transmitted from the connection network to the partial network, for forwarding the audio signal from a transmission channel used for carrying the audio signal in the connection network to another transmission channel to be used for carrying the audio signal in the partial network.