A method includes forming an insulating film over a substrate by introducing a cyclic siloxane compound having a cyclic siloxane as a skeleton and having at least one volatile hydrocarbon group bonded to a side chain, and a silicon-containing compound into a plasma, and converting the insulating film to a porous insulating film by adding energy to the insulating film. The silicon-containing compound is decomposed using less energy as compared with the skeleton of the cyclic siloxane compound, the volatile hydrocarbon group, and the bond between the cyclic siloxane compound and the volatile hydrocarbon group.