1. A feature quantity selection method comprising: a step (a) of clustering a plurality of training images for each of a plurality of combination patterns of a plurality of feature quantities that an image has; and a step (b) of selecting, from among the plurality of combination patterns, a classification-use combination pattern to be used in image classification, based on a result of the clustering, wherein the clustering is performed based on degrees of similarity between the training images that have been calculated with use of the feature quantities constituting the combination patterns.