A capacitor element of a semiconductor device used for a super-LSI is formed by the steps including (a) removing a natural oxide film on a surface of a lower electrode of polysilicon, (b) forming on the surface of the lower electrode an impurity-doped tantalum oxide film, and (c) forming an upper electrode with at least a bottom thereof constituted by titanium nitride. The steps may further include (d) nitriding the surface of the lower electrode after the removal of the natural oxide film, and (e) densifying the tantalum oxide film by way of a high temperature heat treatment after the formation of the tantalum oxide film. In this way, it is possible to reduce thickness of a capacitive insulating film and to form the capacitor element in which the leakage current characteristics are improved.