The improvement of the operation speed by refinement of the integrated circuit is indispensable in order to deal with the increase in the information quantity with the progress of advanced information-oriented society. Signal propagation delay time must be shortened for the speedup of the propagation velocity, and that requires the lowering of dielectric constant of a layer insulation film. The lowering of dielectric constant can make the insulation layer thinner, and shorten the length of wiring. In this study, we analyze theoretically dielectric constant of the siloxane molecule and its derivatives based on ab initio quantum chemical HF and B3LYP methods. Siloxane and its derivatives are expected as low-k layer insulation membrane material at present. The derivatives considered are halogen-substituted siloxanes in order to reduce molecular polarizability and those with pentagonal or hexagonal rings in order to increase the vacancy inside molecules. In order to analyze porosity of the system, we considered a vacancy inside the system that is not counted in ordinary Monte Carlo volume calculations. We discussed the dielectric constant of siloxane and its derivatives not only based on the electric component of polarizability but also on the ionic component that originates from molecular vibrations.