An I/O unit includes a plurality of ports, a mixer includes processing channels, and the mixer and the I/O unit are interconnected via a network. The mixer stores channel data per processing channel. The channel data includes virtual port data for indirectly controlling signal processing of a port to which the channel is patched. The I/O unit stores real port data for controlling a signal processing in a corresponding port per port. In response to a connection instruction, one port and processing channels are interconnected, and real port data of the one port connected with the processing channels is set to a value of virtual port data being maintained for the processing channels.