A W plug (24) is formed and a W oxidation preventing barrier metal film (25) is formed thereon. After that, an SiON film (27) thinner than the W oxidation preventing barrier metal film (25) is formed and Ar sputter etching is performed on the SiON film (27). As a result, the shape of the surface of the SiON film (27) becomes gentler and deep trenches disappear. Next, an SiON film (28) is formed on the whole surface. A voidless W oxidation preventing insulating film (29) is composed of the SiON (28) film and the SiON film (27).