This study evaluated Cs mobility in soil from the viewpoint of the inhibition of Cs sorption onto vermiculite by dissolved soil organic matter (DOM) and the Cs sorption onto DOM. Cs sorption isotherms obtained using soil solution and the diluted soil solution to know the effect of DOM showed that inhibition rate of Cs sorption onto vermiculite was 39.0% in the presence of DOM when Cs concentlation in liquit phase was 10^-2mg/L. In addition, to evaluate the Cs mobility as DOM-sorbed Cs in the soil environment, the Cs sorption onto DOM was investigated by three-dimensional fluorescence analysis and experiments using an anion exchange resin. The distribution coefficient of Cs between DOM and liquid phase was 3.08·10⁴L/kg, which is much larger than those of coexisting elements such as Ca and Mg. It suggested that Cs has higher selectivity for sorption onto DOM than Ca and Mg, and the amount of sorbed Cs onto DOM would be easily mobile in soil.