1. A feature extraction device comprising: an obtaining unit configured to obtain image data having a plurality of pixels; a pixel feature calculation unit configured to calculate first pixel features and second pixel features of each of the pixels, the first pixel features and the second pixel features being different from each other; a classification unit configured to classify a pair of pixels by using the first pixel features for at least some of the plurality of pixels; and a co-occurrence frequency calculation unit configured to calculate a co-occurrence frequency representing a frequency of co-occurrence of the second pixel features of the first pixel and the second pixel features of the second pixel for the set for which a result of the classification by the classification unit is consistent.