1. A method of learning a classifier for classifying an image, comprising: performing a first process, in which a coarse class classifier, configured with a first neural network, classifies a plurality of images, comprising a set of images each having attached a label indicating a detailed class into a plurality of coarse classes including a plurality of detailed classes and then learns a first feature that is a common to each of the coarse classes; and performing a second process in which a detailed class classifier classifies the set of images into a plurality of detailed classes and then learns a second feature that is common to each of the detailed classes, the detailed class classifier configured with a second neural network that is the same in terms of layers other than a final layer, and is different in terms of the final layer, from the first neural network that performs the learning in the first process, wherein at least one of the first process and the second process is performed by a processor.