Plasma processing device or plasma processing method

In order to provide a plasma processing device or a plasma processing method having improved processing yield, a plasma processing device that processes a film layer to be processed on a wafer disposed in a processing chamber in a vacuum container using a plasma formed in the processing chamber is provided with: a detector for detecting the intensity of first light of a plurality of wavelengths in a first wavelength range that is obtained by receiving light emitted from a light source disposed outside the processing chamber into the processing chamber and reflected by an upper surface of the wafer during processing of the wafer, and second light of a plurality of wavelengths in a second wavelength range of the light transmitted from the light source without passing through the processing chamber; and a determination unit which determines a remaining film thickness of the film layer by comparing the intensity of the first light obtained during the processing and corrected using a rate of change in the intensit