An analytical procedure using inductively coupled plasma mass spectrometry (ICP-MS) was developed for determination of very low-level in seawater. Technetium in seawater was reduced and coprecipitated with iron hydroxide. The reduction of Tc by K2S2O5 at pH 4 was rapid and quantitative, and its coprecipitation with iron hydroxide at pH 9 was quantitative. A repetitive coprecipitation method using the same iron as coprecipitater showed no decrease of Tc recovery. Recovery of Tc in various steps of the procedure was examined, i.e., solvent extraction, anion and cation exchange. Background counting rate, with reference to ICP-MS, was extensively examined for various kinds of acids and reagents. The procedure developed showed good recovery for '^Tc and low background counting rate at m/z 99 by ICP-MS. The detection limit was about 40 times lower than the conventional radioanalytical method. The present procedure was applied to determination of "Tc concentration in coastal seawater.