PURPOSE: To always and newly produce a designated window regardless of a fact whether the number of existing windows reach a prescribed level or not by preparing a window number limit releasing part.

CONSTITUTION: A window production request part 1 has a request for production of a new window and a window number limit deciding part 2 decides whether the number of existing windows reach a prescribed level or not. If so, a window number limit releasing part 3 erases selectively one of those existing windows to decrease the number of windows less than the limit level. Then a window production part 4 produces newly a designated window. Thus it is possible to always a new window as designated regardless of a fact whether the number of existing windows reach a prescribed level or not.