Four all-sky CCD imagers have been simultaneously operated at Shigaraki (34.9¿N, 136.1¿E) and Murou (34.6¿N, 136.0¿E), Japan from January to March 1998 to obtain OH and OI (557.7 nm) nightglow images. We determined height of observed small-scale wave structures (horizontal wavelengths of ~7--9 km) by a triangulation technique. For an event of January 26, the structures seen in both nightglow images are estimated to have existed at around 91 km, suggesting that the structures occur at altitudes where these emission layers overlap. For another event of January 30, the structures with a height of 82 km were observed only in the OH images. These results indicate that the height extent of the small-scale wave structures is fairly small, suggesting that such structures are instability features rather than gravity waves. Even if we assume that the structures are gravity waves, their vertical wavelengths estimated by using the dispersion relation are very small.