By appropriately setting the conditions such as the concentration and the flow rate of the etching gas and also the pressure in the system, the etching rate of the P--SiN 44 and that of the P--SiO 45 can be made substantially the same and, in this way, the P--SiN film 44 and the P--SiO film 45 as the interlayer insulating films are caused to be etched uniformly without leaving any undesirable topo