A method is designed for controlling windows of a mixer device including a
processing unit, an operating panel, and a
touch panel provided on the operating panel, the
touch panel functioning as both a display part for displaying windows and an input part for inputting instructions. The method is performed by the
processing unit as follows. First, a determining step determines whether or not any window has already been displayed on the
touch panel when an instruction to display a new window is made. Then, a first displaying step displays the new window when no window has already been displayed, and otherwise displays, when a window has already been displayed, the new window at foreground while displaying at least a
title bar of a window that has already been displayed. Further, a second displaying step displays a close button only on a foreground window among windows displayed on the touch panel and deactivates display of a close button for each window behind the foreground window.