A plasma processing method performed in a plasma processing apparatus having a chamber is provided. This method comprises: (a) providing a substrate having a film stack including a silicon oxide film and a silicon nitride film onto a substrate support in the chamber; and (b) forming a plasma from a processing gas containing HF gas and at least one of CxFy gas (where x and y are integers equal to or greater than 1) and phosphorus-containing gas to etch the film stack, wherein, in (b), the substrate support is controlled to a temperature of 0° C. or more and 70° C. or less, and a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied to the substrate support.