A method for forming a silicone polymer insulation film having low relative dielectric constant, high thermal stability and high humidity-resistance on a semiconductor substrate is applied to a plasma CVD apparatus. The first step is vaporizing a silicon-containing hydrocarbon compound expressed by the general formula Si&agr;O&bgr;CxHy (&agr;=3, &bgr;=3 or 4, x, and y are integers) and then introducing the vaporized compound to the reaction chamber of the plasma CVD apparatus. The next step is introducing additive gas into the reaction chamber. The residence time of the material gas is lengthened by reducing the total flow of the reaction gas, in such a way as to formed a silicone polymer film having a micropore porous structure with low relative dielectric constant.