Method of manufacturing a semiconductor device

A method of manufacturing a semiconductor device, comprising:

• forming a first thermal oxide film serving as a gate oxide film of a MOS transistor on a surface of a silicon substrate having a light receiving element forming region and a MOS transistor forming region;
• forming a polysilicon film on the first thermal oxide film;
• patterning the polysilicon film, to thereby form a gate electrode of the MOS transistor in the MOS transistor forming region;
• removing the first thermal oxide film other than the first thermal oxide film under the gate electrode;
• forming a second thermal oxide film on the surface of the silicon substrate;
• performing ion implantation of an impurity on the light receiving element forming region through the second thermal oxide film, to thereby form an impurity region;
• forming an insulating film on the second thermal oxide film;
• and
• performing anisotropic etching in a state in which a mask layer is formed on the second thermal oxide film in the light receiving element forming region, such that a side wall comprising the insulating film is formed on a side surface of the gate electrode of the MOS transistor and the insulating film remains in the light receiving element forming region.