The present invention provides a technology capable of reducing an area occupied by a nonvolatile memory while improving the reliability of the nonvolatile memory. In a semiconductor device, the structure of a code flash memory cell is differentiated from that of a data flash memory cell. More specifically, in the code flash memory cell, a memory gate electrode is formed only over the side surface on one side of a control gate electrode to improve a reading speed. In the data flash memory cell, on the other hand, a memory gate electrode is formed over the side surfaces on both sides of a control gate electrode. By using a multivalued memory cell instead of a binary memory cell, the resulting data flash memory cell can have improved reliability while preventing deterioration of retention properties and reduce its area.