In this paper the relation between the yield of rice-crop and the summer temperature has been determined for each prefecture of Japan. The average yield of rice-crop in each district correlates to the normal summer temperature of the prefecture, and its correlation coefficient was found to be 0.54. The variability of the yield of rice-crop for the twenty years preceeding 1931 and that of the summer temperature were calculated. The ratio of these two quantities depends upon the position of each prefecture, and the average value of these ratios for the whole country is about 50%.