There is provided an etching apparatus in which, without setting the information of the substance and the chemical reaction, a small number of representative wavelengths can be selected from a waveform at a lot of wavelengths, and an analysis process of etching data which needs large man-hours can be eliminated to efficiently set the monitoring of the etching. The etching apparatus includes: a lot/wafer/step-depending OES-data searching/acquiring function 511 for acquiring a plurality of optical emission intensity waveforms along an etching-treatment time axis; a waveform-change-existence judgment function 521 for judging the existence of the change of the plurality of light emission intensity waveforms; a waveform-correlation-matrix calculating function 522 for calculating a correlation matrix between the optical emission intensity waveforms; a waveform classifying function 523 for classifying the optical emission intensity waveforms into groups; and a representative-waveform selecting function 524 for selecting a representative optical emission intensity waveform from the group.