A plasma processing apparatus includes a detector for detecting interference light of multiple wavelengths from a surface of a sample during processing, a pattern comparator for comparing actual deviation pattern data on the interference light obtained at a given time during processing and a plurality of standard deviation patterns corresponding to two or more thicknesses of the film, and calculating a deviation, the standard deviation patterns corresponding to interference light data of multiple wavelengths obtained, before the processing of the sample, for processing of another sample, a deviation comparator for comparing the deviation between the data and a predetermined deviation and outputting data on a thickness of the film of the sample at that time, a recorder for recording, as time series data, the data on the thickness of the film, and an endpoint decision unit.