Abstract: In a 3-D model window display device, depth (Z direction coordinate) quantities are given to all the windows to be displayed, so that they are regarded as arranged in a three-dimensional space. A virtual viewing point is also placed in the three-dimensional space. By projecting the windows onto a projection plane using the virtual viewing point on a real time basis, this device can display the windows which do not have depth quantities, whereby the user can perceive by intuition the overlapped state of the windows in the Z direction or the positional relationship in the virtual three-dimensional space.