1. An image classification apparatus comprising: a first feature extraction unit configured to segment an input image into a plurality of block images each having a predetermined size, and acquire a feature value of each of the plurality of block images; an area segmentation unit configured to segment the input image into K areas by assigning each of the plurality of block images to any one of the K areas based on the feature value, K denoting an integer equal to or greater than two; a second feature extraction unit configured to acquire, based on an area segmentation result of the area segmentation unit, a feature vector whose elements including, the number of adjacent spots in each of areas, the adjacent spots is where two of the block images are adjacent to each other, the each of areas corresponding to an area to which the each of the two of the block images are assigned; or a probability that one of the block images, which is assigned to one of the K areas, is adjacent to another one of the block images, the another one of the block images is assigned to the same area as to which the one of the block images is assigned or is assigned to a different area from the area to which one of the block images is assigned; and a classification unit configured to classify which of a plurality of categories the input image belongs to, based on the feature vector.