An insulation film is formed on a semiconductor substrate by vaporizing a silicon-containing hydrocarbon compound to provide a source gas, introducing a reaction gas composed of the source gas and an additive gas such as an inert gas and oxidizing gas to a reaction space of a plasma CVD apparatus, and depositing a siloxan polymer film by plasma polymerization at a temperature of -50° C.-100° C. The residence time of the reaction gas in the reaction space is lengthened by reducing the total flow of the reaction gas in such a way as to form a siloxan polymer film with a low dielectric constant such as 2.5.