A numberical model is developed, based on a large amount of observations of diffuse radiation taken at various sites in field experiments over rugger terrain. Three main problems are discussed: Computation of diffuse sky radiation toward a tilted plane; effect of protruding topography; and the amount of radiation reflected by the surrounding high ground to the slope in question. Finally, an analysis is made of the distributive characteristics of diffuse radiation over the southern section of the Dabie mountain range, showing good correspondence between the field of diffuse radiation in this rugged area and its topographic field. It is found that a more important role is played by the facing of the slope in winter but by the shading effect in summer.