A dielectric film of a capacitor is formed using the plasma CVD apparatus. A thin Ta layer is deposited on a semiconductor wafer by using Ta(N(CH.sub.3).sub.2).sub.5 gas and H.sub.2 radicals. The thin Ta layer is then oxidized by O.sub.2 radicals to form a thin Ta.sub.2 O.sub.5 layer. An Si.sub.3 N.sub.4 layer is then formed on the Ta.sub.2 O.sub.5 layer by using SiH.sub.4 and NH.sub.3 gases. The Ta.sub.2 O.sub.5 layer and the Si.sub.3 N.sub.4 layer are alternately laminated one upon the other several times to form a dielectric film of laminated structure. The dielectric film can thus have a composition close to the stoichiometric composition and it can be made high in dielectric constant and excellent in withstand voltage.