A semiconductor substrate measuring apparatus includes a light source to generate irradiation light having a sequence of on/off at a predetermined interval, the light source to provide the irradiation light to a chamber with an internal space for processing a semiconductor substrate using plasma, an optical device between the light source and the chamber, the optical device to split a first measurement light into a first optical path, condensed while the light source is turned on, to split a second measurement light into a second optical path, condensed while the light source is turned off, and to synchronize with the on/off sequence, and a photodetector connected to the first and second optical paths, the photodetector to subtract spectra of first and second measurement lights to detect spectrum of reflected light, and to detect plasma emission light emitted from the plasma based on the spectrum of the second measurement light.