1. An image display device for projecting light on a projection surface to display an image on the projection surface, the image display device comprising: at least one processor; an aptitude detection part configured to detect an aptitude as the projection surface; and memory storing computer-readable instructions that, when executed, cause the at least one processor to function as: a projection control unit configured to control the projection light to allow the image to be displayed on the projection surface to fall within a predetermined projection region in response to a result of detection by the aptitude detection part; and a region selection part configured to select the projection region in response to the result of detection by the aptitude detection part, wherein the aptitude detection part is configured to detect, in a state where the projection region is formed of a mass of a plurality of divided regions, an aptitude value as the projection surface in each divided region, wherein the region selection part is configured to select, based on the aptitude values, one or more divided regions from the plurality of divided regions as the projection region, and wherein the projection control unit is configured to control the projection light to allow the image to be displayed on the projection surface to fall within the selected projection region, wherein the aptitude detection part is configured to detect, based on reflection waves of an electromagnetic wave or an ultrasonic wave radiated toward each divided region, at least one of a position of a reflection point of the electromagnetic wave or the ultrasonic wave and a surface shape of a reflection surface including the reflection point, and the aptitude detection part is configured to detect, based on at least the position of the reflection point or the surface shape of the reflection surface, the aptitude value of the corresponding divided region, wherein the aptitude detection part is configured to treat the projection region as a mass of three or more divided regions and is configured to detect the aptitude value based on at least the position of the reflection point, is configured to set positions determined based on the positions of one or more reflection points within the divided region as evaluation-use positions in the corresponding divided region, is configured to select three or more divided regions, and when the evaluation-use positions in the three or more selected divided regions are on one plane, is configured to detect the aptitude values determined as projection candidate regions with respect to three or more selected divided regions.