In a plasma processing apparatus comprising a processing chamber arranged in a vacuum chamber, a sample stage arranged under the processing chamber and having its top surface on which a wafer to be processed is mounted, a vacuum decompression unit for evacuating the interior of the processing chamber to reduce the pressure therein, and introduction holes arranged above said sample stage to admit process gas into the processing chamber, the wafer having its top surface mounted with a film structure and the film structure being etched by using plasma formed by using the process gas, the film structure is constituted by having a resist film or a mask film, a poly-silicon film and an insulation film laminated in this order from top to bottom on a substrate and before the wafer is mounted on the sample stage and the poly-silicon film underlying the mask film is etched, plasma is formed inside the processing chamber to cover the surface of members inside the processing chamber with a coating film containing a component of Si.