An insulating film is formed with a plasma film forming apparatus which includes a vacuum vessel with an electromagnetic wave incident face F, first gas injection holes made in the vacuum vessel, and second gas injection holes made in the vacuum vessel farther away from the electromagnetic wave incident face F than the first gas injection holes. For example, a first gas is introduced from a position whose distance from the electromagnetic wave incident face F is less than 10 mm into the vacuum vessel. A second gas including an organic silicon compound is introduced from a position whose distance from the electromagnetic wave incident face is 10 mm or more into the vacuum vessel.