[en] We have developed a precise method for determining Sc in natural water samples by preconcentration neutron activation analysis (PNAA) using a carrier-free radioactive nuclide (⁴⁷Sc) as a yield tracer. The separation of Sc from natural water samples was performed by iron precipitation technique followed by an anion exchange method, which enables more than 88% of Sc recovery. Blank samples indicated that the blank contribution was not negligible for determining Sc in natural samples. As for a surface seawater sample, blank corrections in raw data (real amount + blank) were 58-65% and 29% for the use of the normal polyethylene sheet and the high purity sheet, respectively, for sealing samples in neutron activation. However, the corrected data showed a good agreement irrespective of the polyethylene sheets. Our surface seawater Sc data (0.23-0.34 ng/kg) were comparable or slightly higher than the previous data. On the other hand, the rainwater Sc data (2.49-3.05 ng/kg) were one order lower than the reported data. As for rainwater, we are confident that our data were more reliable than the reported data, because the reported data were considered to be susceptible to contamination. Although further studies are required, we believe our newly developed Sc determination technique could be applied to various natural water samples