We have investigated statistical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed in 630-nm airglow images at two stations, Rikubetsu (43.5¿N, 34.8¿MLAT) and Shigaraki (34.9¿N, 25.4¿MLAT), in Japan for 1998--2000 near the solar maximum period. Most of the observed MSTIDs propagate southwestward in the images. The typical wavelength, velocity, period, and amplitude are 100--300 km, 50--100 m/s, 0.5--1.5 h, and 5--15%, respectively. Seasonal variations in these parameters are not clear. The occurrence rate has a major peak (50--60%) in summer that appears ~2 months earlier at lower latitudes and a minor peak in winter. Similar occurrence characteristics are obtained from midlatitude spread-F signatures using multipoint ionosonde data in Japan, though the coincidence of the spread-F and the MSTIDs in airglow images is only 10--15%.