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 is made to classify a plurality of images given as a set of images each attached with a label indicating a detailed class into a plurality of coarse classes including a plurality of detailed classes and is then made to learn a first feature that is a feature common in each of the coarse classes; and performing a second process in which a detailed class classifier is made to classify the set of images into a plurality of detailed classes and is then made to learn a second feature that is a feature common in 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 as, but different in terms of the final layer from, the first neural network made to perform the learning in the first process.