Abstract: A silicon oxide layer is provided on the surface of a semiconductor substrate. Light is applied to the silicon oxide layer while the silicon oxide layer is removed by etching. Intensity of reflected light is detected to obtain an intensity detection signal. The time that would be required for a decrease in thickness of the silicon oxide layer through etching by half of the wavelength of the light is previously obtained to define a reference time period. Within the intensity detection signal, a component periodically changed with a cycle time period equal to the reference time period is extracted. On the basis of the time change of the value of the component, a termination time of silicon oxide layer removal processing is detected.