Abstract: A surface state monitoring apparatus for monitoring a surface state of semiconductor substrates by infrared spectroscopy at fabrication site of semiconductor substrates. The apparatus comprises an incidence optical system for introducing infrared radiation into a substrate-to-be-monitored, a detection optical system for detecting infrared radiation which has undergone multiple reflections inside the substrate and exited from the substrate, a surface state monitoring means for monitoring a surface state of the substrate based on the detected infrared radiation, a position detecting means for optically detecting a position of the substrate, and a control means for controlling a position and an angle at which infrared radiation to be incident on the substrate, corresponding to the position of the substrate.