To provide a data processing device using a neural network that can suppress increase in the occupied area of a chip. A product-sum operation circuit is formed using a transistor including an oxide semiconductor having an extremely small off-state current. Signals are input to and output from the product-sum operation circuits included in a plurality of hidden layers through comparators. The outputs of the comparators are used as digital signals to be input signals for the next-stage hidden layer. The combination of a digital circuit and an analog circuit can eliminate the need for an analog-to-digital converter or a digital-to-analog converter which occupies a large area of a chip.