When controlling a window stack for managing a stack of windows in displaying a plurality of windows on a display unit (11) by a computer (7), each of the application programs (101, 102, and 103) assigns a window to a group, and the window management program (104) collectively arrange the window as a group so as to determine a stacking order of the window in the group. Thereby, a window display performed by an application displaying a window at the forefront is prevented from being improperly influenced by another application.