A semiconductor device having improved adhesiveness between films composing an interlayer insulating film is presented by providing multilayered films in the interlayer insulating films having film density distribution, in which the film density is gradually changes. A SiOC film is deposited to a thickness of 300 nm via a plasma CVD process, in which a flow rate of trimethylsilane gas is stepwise increased. In this case, the film density of the deposited SiOC film is gradually decreased by stepwise increasing the flow rate of trimethylsilane gas. Since trimethylsilane contains methyl group, trimethylsilane has more bulky molecular structure in comparison with monosilane or the like. Thus, the film density is decreased by increasing the amount of trimethylsilane in the reactant gas.