In a method for producing a semiconductor device, two or more kinds of organic siloxane compound materials each having a cyclic SiO structure as a main skeleton and having different structures are mixed and thereafter vaporized. Alternatively, those two or more kinds of organic siloxane compound materials are mixed and vaporized simultaneously to produce a vaporized gas. Then, the vaporized gas is transported to a reaction furnace together with a carrier gas. Then, in the reaction furnace, a porous insulating layer is formed by the plasma CVD method or the plasma polymerization method using the vaporized gas.