Theoretical considerations concern the relationship between net primary productivity of natural vegetations and climatic conditions. Those suggest that net primary productivity (NPP) of natural vegetations linearly increases with increasing annual net radiation (R_n) and that the proportionality constant between R_n and NPP decreases very rapidly with increment of the value of radiative dryness index (RDI) that is the ratio of R_n to the product of annual precipitation and latent heat of evaporation. NPP data (682 set) of about 260 locations over the world were used to verify the theoretical prediction. A nonlinear regression equation describing the dependence of NPP on R_n and RDI was obtained and named “Chikugo model”. The NPP-distribution map over Japan was made on the basis of the Chikugo model using the climatic data of Japan. It was found that NPP over Japan changes from about 8t DW/(ha yr) in mountain districts of Hokkaido to about 18t DW/(ha yr) in southern coastal areas of Kyushu and Shikoku. These results agreed well with the net primary production of forests obtained by plant ecologists.