The probability distribution of background cancer risk was examined using the detriment-adjusted risk defined by the International Commission on Radiological Protection (ICRP) on the basis of data on background cancer mortality in Japan. The purpose of this study is to gain information that is important in discussing the acceptability of radiation exposure in the low-dose region of 1 mSv/y or lower. The probability distribution was compared with the distribution of maximum assumed risk due to radiation from man-made radioactive nuclides calculated using the representative person concept defined by ICRP. The results revealed that the risk for members of the public is sufficiently small and lower than the risk that is within the deviation of the background cancer risk, if there is appropriate radiation protection compliance with the dose limit of 1 mSv/y.