In a semiconductor device including a split gate type MONOS memory, and a trench capacitor element having an upper electrode partially embedded in trenches formed in the main surface of a semiconductor substrate, merged therein, the flatness of the top surface of the upper electrode embedded in the trench is improved. The polysilicon film formed over the semiconductor substrate to form a control gate electrode forming a memory cell of the MONOS memory is embedded in the trenches formed in the main surface of the semiconductor substrate in a capacitor element formation region, thereby to form the upper electrode including the polysilicon film in the trenches.