Lead-210 dating method was applied to the sediment cores collected in Ago Bay, Mie prefecture, Japan where the seawater and bottom sediments have been contaminated due to the active pearl cultivation carried out for more than 100 years. A contamination with pearl cultivation was observed on the vertical distribution of the organic carbon (OC) and nitrogen (N), and the OC/N ratio suggesting marine plankton was a major source as the organic contaminants. A surface mixed layer (SML) was observed on the excess <sup>210</sup>Pb vertical distribution and the underlying part, below the SML, was divided into two layers with different sedimentation rates, suggesting the change of sedimentary condition in the late 1960s. The transition of the sedimentation rate is supposed to occur in connection with the change of pearl-production activity in Ago Bay. The surface mixed layer of <sup>137</sup>Cs has expanded to the deeper layer than that of the excess <sup>210</sup>Pb and the inventory of <sup>137</sup>Cs was significantly smaller compared to that of the excess <sup>210</sup>Pb. These facts suggest different behavior of <sup>210</sup>Pb and <sup>137</sup>Cs in the marine environment after they were transferred from the freshwater environment as soil-particle associated radionuclides.