PURPOSE: To equalize the amount of ions of gas converted into plasma and to form a high quality insulating film of a uniform thickness by concentrically arranging plural gas introducing holes pierced in the face of a lower electrode confronting a upper electrode so that the intervals are made gradually smaller from the central part toward the peripheral part.

CONSTITUTION: An upper electrode 1 and a lower electrode 2 are used as parallel flat electrodes in a plasma CVD apparatus. Gas is introduced into the apparatus from the central part of the upper electrode 1 through plural gas introducing holes 3 pierced in the faces of the upper electrode 1 confronting the lower electrode 2. The gas introducing holes 3 are concentrically arranged so that the intervals (d) are made gradually smaller from the central part toward the peripheral part.