Means for reducing an arsenic ion concentration in the solution to the degree of ultra trace amount are provided. Ammonium molybdate is supported by a nanostructure material by mixing the nanostructure material, which is obtained after the nanostructure material such as an alumina reacts with a surfactant, in the solution containing the ammonium molybdate. The nanostructure material supporting an arsenic ion adsorption compound such as ammonium molybdate can selectively adsorb and remove trace of arsenic ion in the solution by a room temperature treatment without a water conditioning such as pH control. In our removal system of arsenic, extra posttreatments are not needed because special pretreatments are not carried out, and special heating equipments are not used. Accordingly, our removal system of arsenic can be constructed at low cost. Furthermore, it can supply an arsenic-free solution by be constructed at multi stages.