A method for forming an insulation film having filling property on a semiconductor substrate by plasma reaction includes: vaporizing a silicon-containing hydrocarbon having a Si—O bond compound to provide a source gas; introducing the source gas and a carrier gas without an oxidizing gas into a reaction space for plasma CVD processing; and forming an insulation film constituted by Si, O, H, and optionally C or N on a substrate by plasma reaction using a combination of low-frequency RF power and high-frequency RF power in the reaction space. The plasma reaction is activated while controlling the flow of the reaction gas to lengthen a residence time, Rt, of the reaction gas in the reaction space.