1. An image management device for ranking a plurality of images based on importance degrees thereof, comprising: an image acquisition unit configured to acquire a plurality of images; a detection unit configured to detect, in each of the images acquired by the image acquisition unit, any objects included therein; an extraction unit configured to extract image feature values of each detected object; a clustering unit configured to categorize each detected object into one of a plurality of clusters based on the image feature values of the detected objects; a block generation unit configured to generate a plurality of blocks by grouping the images acquired by the image acquisition unit into blocks; an intra-block importance degree calculation unit configured to calculate, 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; a cluster importance degree calculation unit configured to calculate 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 an image importance degree calculation unit configured to calculate an image importance degree of each of the images based on the calculated cluster importance degrees.