To improve the manufacturing yield of a semiconductor device by preventing the occurrence of stringers by avoiding the occurrence of such a state that the surface of an element isolating film becomes lower in height than that of the substrate of an element section.

An element isolating film 16 is formed by forming a second thermally oxidized silicon film at the place of a recessed section 14, and a second photoresist film 32 having a flat surface is formed on the film 16 and a nitride film 6 (Fig.1 (C)). Then, the surface of the substrate is flattened by dry-etching (resist etching back) the photoresist layer 32, nitride film 6, and element isolating film 16 until a first thermally oxidized film 4 is exposed (Fig.1 (D)). In addition, the first thermally oxidized film 4 and the surface section of the element isolating film 16 are wet-etched until the film 4 is removed by using an etchant containing a hydrogen fluoride (Fig.1 (E)) and, after a pressure-reduced oxide film 30 is formed on the etched surface, a polysilicon layer 36 which is used as a resistance element or transistor element is formed.