One or more input channels are selected from among a plurality of input channels, and signals of the selected one or more input channels are mixed into a first output channel. A particular input channel to be excluded from the plurality of input channels is set as desired. Then, a setting is made such that the signals of the selected one or more input channels, having the signal of particular input channel excluded therefrom, are mixed into a second output channel. To the first output channel are set such signal paths as to allow mixing of all the signals of the selected one or more input channels. To the second output channel are set such signal paths as to allow mixing of the signals of the selected one or more input channels with the signal of the particular input channel excluded thereform.