A mask layer and a to-be-processed layer are irradiated with light to measure interference light formed of reflected lights from the mask layer and reflected lights from the to-be-processed layer. Thereafter, an interference component brought by the mask layer is removed from the waveform of the measured interference light, thereby calculating the waveform of the interference light brought by the to-be-processed layer. The thickness of the remaining to-be-processed layer is determined on the basis of the calculated waveform of the interference light and the thickness of the remaining to-be-processed layer is compared with a desired thickness thereof. In this way, an end point of processing on the to-be-processed layer is detected.