A stable etching process is realized at an earlier stage by specifying the combination of wavelength and time interval, which exhibits a minimum prediction error of etching processing result within a short period. For this, the combination of wavelength and time interval is generated from wavelength band of plasma emission generated upon etching of the specimen, the prediction error upon prediction of etching process result is calculated with respect to each combination of wavelength and time interval, the wavelength combination is specified based on the calculated prediction error, the prediction error is further calculated by changing the time interval with respect to the specified wavelength combination, and the combination of wavelength and time interval, which exhibits the minimum value of calculated prediction error is selected as the wavelength and the time interval used for predicting the etching processing process.