A data device with a small circuit area and reduced power consumption is used. The data processing device includes a NAND memory portion and a controller. The memory portion includes a first string and a second string in different blocks. The first string includes a first memory cell, and the second string includes a second memory cell. On reception of first data and a signal including an instruction to write the first data, the controller writes the first data to the first memory cell. Then, the controller reads the first data from the first memory cell and writes the first data to the second memory cell.