A plasma processing apparatus is provided using a method of measuring the thickness of a processed material, by which the actual remaining thickness or etching depth of a processed layer can be correctly measured online. The plasma processing apparatus includes a detector 11 for detecting interference light of multiple wavelengths from a surface of a sample during processing, pattern comparing means 15 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, deviation comparing means 115 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, remaining-thickness time-series data recording means 18 for recording, as time series data, the data on the thickness of the film, and an endpoint decision unit 230 for deciding completion of a predetermined amount of etching by using the data on the thickness of the film.