A method for forming an organic/inorganic hybrid insulation film includes the following steps. An organic silicon compound containing siloxane bonds is vaporized, the vaporized organic silicon compound is transported to a reaction chamber maintaining the compound in a monomer state, and then, the organic/inorganic hybrid insulation film having a main chain structure where siloxane parts and organic molecule parts are alternately combined on a substrate installed in the reaction chamber is formed by plasma-polymerizing the vaporized organic silicon compound in the reaction chamber.