In a digital mixer including a display, cursor controls, an increase/decrease control, and a plurality of channel strips for controlling parameters of input channels associated therewith, on a control panel, the channel strips each having a selection switch, an assignment switch is provided for assigning any one parameter among the parameters of the input channel to the increase/decrease control, so that where operation of the increase/decrease control is detected, when no selection switch has been operated, a value of a parameter displayed at the position of the cursor is changed in accordance with the operation of the increase/decrease control, while when any selection switch has been operated, a value of a parameter assigned to the increase/decrease control among parameters of an input channel corresponding to a channel strip having the selection switch is changed.