A film forming method is characterized in that the method is provided with a step of introducing a processing gas including inorganic silane gas into a processing chamber, in which a mounting table composed of ceramics including a metal oxide is arranged, and precoating an inner wall of the processing chamber including a surface of the mounting table with a silicon-containing nonmetal thin film; a step of mounting a substrate to be processed on the mounting table precoated with the nonmetal thin film; and a step of introducing a processing gas including organic silane gas into the processing chamber, and forming a silicon-containing nonmetal thin film on a surface of the substrate mounted on the mounting table.