PURPOSE: To uniformly deposit the protective coating of an element by forming the film with the positive column plasma of reactive gases generated in a meshed container between plural pieces of rf-electrodes recessed toward the inner side.

CONSTITUTION: Two pieces of the rf-electrodes 1, 2 are formed to recess toward the inner side. The container 3 consisting of a meshed conductor which is recessed toward the inner side is provided between these electrodes 1 and 2. The film is formed by the positive column plasma of the reactive gases generated in the container 3. The opposite surfaces of the rf-electrodes 1, 2 and the container 3 are formed to approximately spherical or cylindrical surfaces. A contribution is made to the improvement in the reliability of the element in this way.