The etching reaction of a photochemically deposited silicon nitride film with F2 has been observed in situ using polarization-modulation infrared spectroscopy and quadruple mass spectrometry. The infrared spectrum of the silicon nitride film before etching exhibited two bands at 1030 and 975 cm-1, arising from Si-N vibration. Exposure of the film at 423 K to F2 led to an intensity decrease of the lower-frequency band, while the higher-frequency band increased. Simultaneous mass analysis revealed that the etching products evolved into the gas phase were SiF4 and H2. However, a further admission of F2 resulted in a slight decrease in intensity of the 975 cm-1 band as well as a slight evolution of SiF4. These results strongly suggest the presence of a metastable Si-rich layer on the surface of the silicon nitride film prior to reaction with F2. Infrared measurements have also been made in the Si-H stretching region, the results of which are described and discussed.