In a plasma processing method and apparatus for processing a film to be processed contained in a film structure preliminarily formed on an upper surface of a wafer mounted in a processing chamber, by using plasma, a residual film thickness at an arbitrary time is calculated using a result of comparing detective differential waveform pattern data with actual differential waveform pattern data. The detective differential waveform pattern data is produced by using two basic differential waveform pattern data which respectively use, as parameters, residual thicknesses of the films to be processed in film structures having underlying films with different thicknesses and the wavelengths of the interference light. The detective waveform pattern data being preliminarily prepared prior to processing of the wafer. Determination is made as to whether or not an object of the processing has been reached by using the residual film thickness.