There is provided a plasma processing method performing a plasma etching process on an oxide film of a target substrate through one or more steps by using a processing gas including a CF-based gas and a COS gas. The plasma processing method includes: performing a plasma etching process on the oxide film of the target substrate according to a processing recipe; measuring a concentration of sulfur (S) remaining on the target substrate (residual S concentration) after the plasma etching process is performed according to the processing recipe; adjusting a ratio of a COS gas flow rate with respect to a CF-based gas flow rate (COS/CF ratio) so as to allow the residual S concentration to become equal to or smaller than a predetermined value; and performing an actual plasma etching process according to a modified processing recipe storing the adjusted COS/CF ratio.