In a method of forming an interlayer insulating film by plasma CVD, an organic siloxane compound including one or more silicon atoms each having at least three or more units each represented by a general formula, —O—Si(R1R2)—OR3 (wherein R1 and R2 are the same as or different from each other and are a methyl group, an ethyl group or a propyl group, and R3 is the same as or different from R1 and R2 and is a methyl group, an ethyl group, a propyl group or a phenyl group) is used as a raw material.