The purpose of this study is to clarify the diurnal variation of global radiation and diffuse radiation on the hill slopes of Nihon Davos in the Sugadaira Plateau, Nagano Prefecture. The locations of the five observation points are shown in Fig. 1. The observation was held from 6:20 a.m, to 4:50 p.m. on the 4th of November, 1978 and the weather condition during this period was perfect cloudless. The result of the observation are summarized as follows: 1) The global radiation on a horizontal surface on the top of the hill is larger than those on other slopes all day (Fig. 7). The probable cause may be due to the higher ratio of sky to the hemisphere at the top than those at other points (Table 1). 2) Excepting the northern point, four points have two peaks in the diurnal variation of diffuse radiation. The first peak occurred early in the morning on the top of the hill and the eastern slope but it appeared before noon on the southern slope and western slope. On the other hand, the diffuse radiation on the northern slope has only one peak after sunrise and after that time it is very low and constant until sunset (Fig. 8). 3) As a result of the observation held on cloudless day, the diurnal variations of global radiation on the sloping surfaces are similar to the usual theoretical results of direct radiation on sloping surfaces (Fig. 9).