When a semiconductor wafer placed in a chamber and having films thereon is etched
using plasma generated in the chamber, a change in the amount of lights with at
least two wavelengths, obtained from the wafer surface during the processing, is
detected. The time between the time at which the amount of a light with one of
two wavelengths is maximized and the time at which the amount of a light with the
other wavelength is minimized is compared with a predetermined value to determine
the state of etching processing.