1. An image management device for ranking a plurality of images based on importance degrees thereof, comprising: a processor; and a non-transitory memory having stored thereon executable instructions which, when executed, cause the processor to perform: acquiring a plurality of images; detecting, in each of the acquired images, any objects included therein; extracting image feature values of each detected object; categorizing each detected object into one of a plurality of clusters based on the image feature values of the detected objects; generating a plurality of blocks by grouping images in which an object is detected; calculating, for each of the blocks, an intra-block importance degree of each of the clusters to which an object in an image in the block belongs based on a relationship between the image and the cluster; calculating a cluster importance degree of each of the clusters in the images by accumulating the intra-block importance degrees calculated for each of the clusters in each of the blocks; and calculating an image importance degree of each of the images based on the calculated cluster importance degrees.