A method of statistical analysis is used to study parameters of the medium-scale internal gravity waves (IGWs) from the data of the spectral airglow temperature imager (SATI) observations of the OH and O₂ nightglow emissions at Shigaraki, Japan (35°N, 136°E) during April 1998 to May 2001. Average relative magnitudes of spectral components of airglow variations are 1.8–2.4% for the emission rates and 0.5–1% for the rotational temperature in different seasons. Distributions of periods, horizontal wavelengths, and phase speeds have the main maxima at 0.5–2 hours, 100–500 km, and 50–150 m/s, respectively. In winter, the distributions of propagation azimuths are broad with a dominance in the northwest sector and in the southern half of the sky for OH and in the northwest and southeast sectors for O₂. In spring and summer, IGWs propagate mainly to the northern half of the sky with larger occurrence in the northeast sector for OH emission. Seasonal variations of the average and median IGW magnitudes do not exceed 20–30% and are not stable. In winter, IGWs have maximum occurrence with larger magnitudes, horizontal wavelengths, and phase speeds, particularly at azimuths of 90–210 deg. These results are compared with previous studies.