1. A feature extraction device comprising: a memory that stores a computer executable unit; a processor configured to execute the computer executable unit stored in the memory; an obtaining unit stored in the memory and executed by the processor, and configured to obtain image data having a plurality of pixels; a pixel feature calculation unit configured to calculate two features for each of the pixels, the two features being a first feature and a second feature of each of the pixels, the first feature and the second feature being different from each other; a classification unit configured to classify pairs of pixels by using the first feature into sets as classified sets for at least some of the plurality of pixels, each pair including a first pixel and a second pixel; and a co-occurrence frequency calculation unit configured to calculate a co-occurrence frequency representing a frequency of co-occurrence of the second feature of the first pixel and the second feature of the second pixel for pairs belonging to a same classified set.