The invention enhances the accuracy of an end point detection when an insulation film formed on a semiconductor substrate is dry-etched. Gate layers made of polysilicon are formed, and an end point detection dummy layer made of polysilicon is formed on a LOCOS. After the gate layers and the dummy layer are formed, a TEOS film is formed on a silicon substrate so as to cover the gate layers and the dummy layer. The TEOS film, a thin gate oxide film and a thick gate oxide film are then dry-etched to form sidewalls on the sidewalls of the gate layers and also expose the front surface of the P well of the silicon substrate in a region surrounded by the LOCOS. The end point detection dummy layer helps the end point detection by being exposed during this dry-etching to enhance the accuracy of the end point detection.