PURPOSE: To more easily manufacture the title semiconductor device in element isolation structure having higher reliability.

CONSTITUTION: A silicon dioxide film, a silicon nitride film and another silicon dioxide film are successively deposited on a semiconductor silicon substrate 1 to form an ONO film 13. Next, a shield plate electrode 4, the other silicon dioxide film 5 are successively formed on this ONO film 13 to form sidewalls 6 comprising silicon dioxide film on the sidewall parts of the shield plate electrode 4 and the silicon dioxide film 5. Next, a gate electrode 8 is formed on the silicon dioxide film 5 and the ONO film 13. Through these procedures, the reliability upon the title semiconductor device can be enhanced due to the low defective density of the ONO film 13 used as a dielectric film of the shield plate electrode 4 as well as the steps can be cut down due to the ONO film 13 used as the common dielectric film of the shield plate electrode 4 and the gate electrode 8.