Once bookmark registration is instructed by user operation while any detailed setting window is being displayed on a background screen on a display of a digital mixer, only the currently-displayed detailed setting window is bookmark-registered. Thus, once bookmark call operation is performed while a desired background screen is being displayed, the currently-displayed background screen is maintained as-is, and the called detailed setting window is displayed on the background screen. Further, bookmark registration can be made in association with a mixer channel, and, in response to bookmark call operation, parameters of the associated channel can be called and displayed.