A window system is constructed by a checking section to check an overlapping state of a plurality of windows displayed on a display, a forming section to form window graphs each showing the positional relation of each window when the windows are seen from another direction such as vertical or horizontal direction different from the overlapping direction and each corresponding to each window, a display such as a color CRT or LCD display to display the window graphs, a detector to detect an overlapped portion of the window graphs from the window graphs formed, and a changing section to change an attribute of the window graph to clarify the overlapped portion of the window graphs when such a portion is detected. By performing a selecting or operating instruction by a mouse cursor or the like to a desired window graph, the window corresponding to the instructed window graph can be selected or operated.