[en] The PCDDs/DFs contamination in the environment has concerned our society even after prohibition of its use. Dr. Sakai et al. showed the change in concentration of PCDDs/DFs historically in sediment core samples taken from Osaka Bay and Lake Biwa. Sediment core samples were taken from five points on urban coastal area in Osaka Bay and three points on the coastal area in Lake Biwa. We measured the concentration of ²²⁶Ra, ²¹⁰Pb, ²²⁸Th, ⁴⁰K and ¹³⁷Cs in the sediment core samples by using gamma-ray spectrometry and estimated the sedimentation rate with the ²¹⁰Pb method. The sedimentation rate was about (200-600) mg/cm²·y in Osaka Bay and (150-300) mg/cm²·y n Lake Biwa. The characteristics on distribution of the ²²⁶Ra, ²¹⁰Pb, ²²⁸Th, ⁴⁰K and ¹³⁷Cs concentration in the surface sediments on the cost of Osaka Bay and Lake Biwa were also investigated. The peaks of ¹³⁷Cs concentration (35-70 Bq/kg) in three core samples from Lake Biwa were clearly observed. The sedimentation rates with ¹³⁷Cs distribution indicated similarity to those calculated from the ²¹⁰Pb method in the samples of Lake Biwa. However, a remarkable peak of ¹³⁷Cs concentration was not observed in the core samples taken in Osaka Bay. These concentration were under 6 Bq/kg. We have kept on measuring the annual deposition rate (about 100 Bq/m²) of ²¹⁰Pb in the Institute in Osaka with observation of the monthly deposition using a basin. We estimated the area contributed to the points taken core samples in comparison the annual deposition rate of ²¹⁰Pb at the sampling points in Osaka Bay and Lake Biwa with that mentioned above. The mineral composition of the core samples were also measured by X-ray diffraction analysis. It is possible that the distribution of the concentration of these nuclides and the mineral composition in the sediments shows the environmental change. (author)