To effectively utilize a memory by controlling the number of windows which can be opened on the multiwindow system in subsystem units.

When a new window is opened on a subsystem 1 of the multiwindow system which is divided into subsystems, the number of windows which are already open on the subsystem 1 is inquired of a window number control means 5. When the number already reaches the maximum number of windows that can be opened on the subsystem 1, the window number control means 5 opens closure-selected windows 9 and makes an operator select unnecessary windows, which are closed. After the operator confirms the closure, this window ends. Then the new window requested to be opened is opened.