A neural network device comprises a processor that performs an operation for training a neural network, a feature extraction module that extracts unlabeled feature vectors that correspond to unlabeled images and labeled feature vectors that correspond to labeled images, and a classifier that classifies classes of query images, wherein the processor performs first learning with respect to a plurality of codebooks by using the labeled feature vectors, and performs second learning with respect to the plurality of codebooks by optimizing an entropy based on all of the labeled feature vectors and the unlabeled feature vectors.