A plasma processing apparatus can prevent a sheath from becoming distorted, simplify a configuration of the apparatus, and prevent particles from attaching to a substrate. The plasma processing apparatus performs plasma processing on the substrate. A housing chamber houses the substrate. A mounting stage is disposed within the housing chamber and mounted with the substrate. An annular member is disposed in the mounting stage. A power supply unit supplies high-frequency power to the mounting stage. An observation unit optically observes the distribution of the plasma. A voltage applying unit applies a DC voltage to the annular member. A control unit sets the value of the DC voltage to be applied based on the observed plasma distribution.