The cloud-base topography of cumulus humilis clouds over the mountainous terrain and the nearby plain area was measured by the stereo-photogrammetric method in the summer seasons of 1969 to 1971 near Sapporo. As a result of analysis, it was noted that the height of the base of cumulus humilis clouds over the mountainous terrain was considerably higher than that over the lower plain. The res:1lt showed also that the height of cloud base coincided with that of lifting condensation level which was calculated from Henning's formula. The difference in the height of cloud bases over the mountainous terrain and over the lower plain area was quantitatively explained, utilizing the difference in the potential temperature and dew point at these two places. It was finally concluded that the excess height of cumulus humilis cloud base on the mountainous terrain over that on the lower plain area was mainly caused by the dryness of an air layer around 300 m altitude above sea level. The formation condition of cumulus humilis clouds was studied, comparing with those of stratiform clouds and cloudless sky.