Polarization modulation IR spectroscopy has been applied for in situ observation of the growth process of silicon nitride films under photo-chemical vapor deposition conditions. Two absorption bands ascribable to SiN stretching modes were observed. In the initial stage of the film deposition, a band appeared at 965 cm-1, whose intensity was nearly constant during the deposition. As the film was further deposited, a new band emerged around 1030 cm-1 which grew rapidly. Upon UV irradiation in vacuum, the former band decreased in intensity while the latter increased. From these results, the presence of a metastable surface layer, which changes into a stable silicon nitride network upon UV irradiation, is proposed.