Provided is a window display system and the like, in which plural window structures are switched to be displayed on a single window. An event processing program is associated with a first window structure, and this first window structure is assigned to a basic window. By executing an event processing program, a second window structure is assigned to the basic window. By changing or updating the basic window, or by assigning the second window structure to the basic window, a drawing window is drawn. On the single drawing window, the first and second window structures and are switched to be displayed. The drawing of the drawing window is caused by window size change, window movement, change of overlapping window position, window visualization, invalidation of window drawing area, or window color change.