A novel semiconductor device is provided. In reservoir computing using an input layer, a reservoir layer, and an output layer, variation in threshold voltage between transistors is used as a weight used for product arithmetic processing. Two transistors are provided in one product arithmetic circuit and data u is supplied to gates of the two transistors. Drain current of each of the transistors is determined by the data u and the threshold voltage of the transistor. The difference between the drain currents corresponds to a product arithmetic result. The difference between the drain currents is converted into voltage to be output. A plurality of product arithmetic circuits are connected in parallel to form a product-sum arithmetic circuit.