In a hierarchical display structure, a high layer window contains one or more input images that serve as a window switchover region for switching into a low layer window. Each of the input images is a first correspondence image that has a unique color. When a manipulation device selects one of the input images and performs a determination input to the selected input image in the high layer window being displayed on a screen of a display device, the display on the screen is switched from the high layer window into the low layer window. The low layer window on the screen contains a background image as a second correspondence image that has a color identical to a color of the input image to which the determination input was performed in the high layer window.