Manually-operable and automatically-driveable fader operators are provided in corresponding relation to a plurality of channels, and a plurality of the fader operators are grouped in such a manner that the fader operators in a same group are operable collectively in an interlocked fashion. On a display screen, a window of a given group is opened in response to selection operation by a user, and a graphic image indicative of settings of the individual fader operators belonging to the given group is displayed. In response to operation by the user on the image displayed on the window of the given group and indicative of the settings of the individual fader operators, an image for effecting a change in the settings is displayed, and the individual fader operators belonging to the given group are automatically driven, to thereby change the settings. Such arrangements allow a grouping state of the operators to be identified visually with an increased ease and also facilitate setting operation.