The method comprises forming a first silicon nitride film covering a pixel circuit section by thermal CVD; forming an opening in the first silicon nitride film by removing a first portion of the first silicon nitride film while remaining a second portion of the first silicon nitride film; forming a second silicon nitride film covering the opening by plasma CVD; forming an insulating film covering the first silicon nitride film and the second silicon nitride film and covering a peripheral transistor in the peripheral circuit section; and forming a contact plug passing through the insulating film and being in contact with the peripheral transistor.