A siloxan polymer insulation film has a dielectric constant of 3.1 or lower and has —SiR2O— repeating structural units with a C atom concentration of 20% or less. The siloxan polymer also has high thermal stability and high humidity-resistance. The siloxan polymer is formed by directly vaporizing a silicon-containing hydrocarbon compound of the formula Si&agr;O&agr;−1R2&agr;−&bgr;+2(OCnH2n+1)&bgr; wherein &agr; is an integer of 1-3, &bgr; is 2, n is an integer of 1-3, and R is C1-6 hydrocarbon attached to Si, and then introducing the vaporized compound with an oxidizing agent to the reaction chamber of the plasma CVD apparatus. The residence time of the source gas is lengthened by reducing the total flow of the reaction gas, in such a way as to form a siloxan polymer film having a micropore porous structure with low dielectric constant.