A method of forming an interlayer insulating film, wherein a material containing, as a main component, an organic silicon compound represented by the following general formula: R1 x Si(OR2)4-x (where R1 is a phenyl group or a vinyl group; R2 is an alkyl group; and x is an integer of 1 to 3) is caused to undergo plasma polymerization or react with an oxidizing agent to form an interlayer insulating