[en] In the previous study, a statistically significant positive correlation between natural background radiation exposure rates and crude (non-age-adjusted) cancer mortality rates was observed in 46 Japanese prefectures over the period from 1968 until 1978. In the present investigation, however, the significance of this correlation mostly disappeared through age adjustment with only the two exceptions of female stomach and uterine cancers. Age adjusted male esophagus cancer mortality rate still showed a significant negative correlation. Female esophagus and pancreas cancers became negatively correlated with exposure rate through age adjustment. It was suggested that natural radiation levels are positively correlated with prefectural population component ratios older than 40, 50 and 65 years, which was considered to be one of the causes of apparent correlation between exposure rates and crude cancer mortality rates. (author)