Once a mixer is set in a predetermined operation, an input-logical-channel selecting section supplies sound signals, input via a cascade input terminal, to an input signal processing section via an input patch section, so that the sound signals can be mixing-processed as sound signals of normal input channels. At the same time, a portion of sound signals input via an input terminal are supplied to mixing buses, so that these sound signals can be handled as cascade-related signals. In accordance with a model of another, or cascaded-to, mixer, arrangements are made such that normal-input/output-channel input terminals can be assigned to cascade input/output purposes. With a block diagram display section indicating what signals the individual input/output terminals are currently assigned to within the mixer, a user can grasp at a glance the current assignment state.