We made sorption experiments of actinoids on Mn oxide in dilute NaCl solution. Sorption affinity of actinoids on synthetic Mn oxide was Th(IV) > U(VI) > Np(V). The specific sorption behavior of actinoids on biogenic Mn oxide was observed. Thorium sorbed on biogenic Mn oxide was desorbed into solution with increasing time. Neptunium was not sorbed on biogenic Mn oxide. The specific sorption behavior is possibly due to organic ligands released from microorganisms and surface properties of biogenic Mn oxide.