A plasma processing apparatus and method with an improved processing yield, the plasma processing apparatus including detector configured to detect an intensity of a first light of a plurality of wavelengths in a first wavelength range and an intensity of a second light of a plurality of wavelengths in a second wavelength range, the first light being obtained by receiving a light which is emitted into the processing chamber from a light source disposed outside the processing chamber and which is reflected by an upper surface of the wafer, and the second light being a light transmitted from the light source without passing through the processing chamber; and a determination unit configured to determine a remaining film thickness of the film layer by comparing the intensity of the first light corrected using a change rate of the intensity of the second light.