In a digital mixer that processes audio signals in a plurality of channels, layer data which indicates a channel to be assigned to each channel strip in a channel strip section can be set to indicate retainment of assignment status for any of the channel strips. When the layer is selected, the digital mixer assigns the channel indicated by the layer data corresponding to the selected layer to each channel strip for which assignment of some channel is indicated, and leaves the assignment of the channel unchanged regarding the channel strip for which retainment of assignment status is indicated in the layer data.