1. A map image display system comprising: a controller configured to: acquire map data that is divided into a plurality of areas and hierarchically structured into a plurality of layers based on an amount of road network information; define a display target area on a display device based on the acquired map data; divide the display target area into a plurality of division areas with parts of neighboring ones of the division areas overlapping each other; generate a map image for each of the division areas, each map image being based on a different layer of the map data; in an area where two or more of the generated map images overlap, gradually change a transmittance of a first one of the overlapped map images so that a transmittance of an outer portion of the first overlapped map image becomes higher than a transmittance of an inner portion of the first overlapped map image; display a second one of the overlapped map images on the display device; and after the second map image is displayed on the display device, display the first map image on the display device so that the first map image is overlapped on the displayed second map image.