In a conventional method for forming a capacity element for DRAM, a tantalum oxide film is formed on the surface of polycrystal silicon film constituting a capacity lower electrode, and a high temperature treatment is then carried out in an oxygen atmosphere to improve leakage current properties, thereby converting this tantalum oxide film. In the capacity element having the thus formed capacity insulating film, an obtainable capacity value is small. In the present invention, a densification treatment is carried out at a relatively low temperature in place of the high temperature treatment step of the tantalum oxide film, whereby the capacity element having the large capacity value can be formed without deteriorating the leakage current properties.