While an AUX-bus selecting operator is being operated beyond a predetermined time period, displays of channel-ON operators, normally indicating ON/OFF states of corresponding input channels, indicate ON/OFF states of signal delivery from the input channels to the AUX bus. Thus, a user can readily confirm, with minimized operation, the ON/OFF states of the signal delivery from the input channels to the second bus. Operating any one of the channel-ON operators in this state can change the ON/OFF state of the signal delivery from the corresponding input channel to the AUX bus. Further, operating the AUX-bus selecting switch while keeping operating the layer operator, current fader settings of the input channels can be copied as delivery levels of signals from the input channels to the AUX bus. Because the delivery levels of sound signals from the input channels to the bus can be set using the fader settings of the input channels as initial values, time and labor necessary for the setting operation can be minimized.