Main and sub assignable controls are provided for each channel strip. Predetermined screens, each including a parameter display, are displayed dividedly or separately on first and second display devices. Thus, any one of a plurality of parameters included in the predetermined screen, being displayed on the first display device, is variably assigned to the main assignable control, while any one of a plurality of parameters included in the predetermined screen, being displayed on the second display device, is variably assigned to the sub assignable control. Further, when a predetermined pop-up screen, including at least a parameter display, is to be further displayed, in response to a screen display switching instruction, over the predetermined screen being displayed on the first display device, a parameter included in the pop-up screen is fixedly assigned to each of the main and sub assignable controls. In this way, the plurality of assignable controls can be used properly in accordance with a screen display.