||As a result of the accident at the nuclear power plant in Fukushima in 2011, a large amount of radioactive Cs ions were released into the water, which greatly affected the aquatic ecosystem. The adsorption method is widely used for the removal of hazardous materials because such method is simple and, thus does not need pretreatment , compared with other methods. Prussian blue (PB) is a typical adsorbents for Cs ions. Cs ions could be removed as trapped in the crystal lattice of PB. But due to the fine structure of PB, leading to difficulty of filtration, the bulk, supporting materials are required. Sodium alginate which can gellate by cross-linking with suitable divalent or trivalent metal ions are used. We prepared sodium alginate-based crosslinked gel by synthesizing PB inside the sodium alginate gel. After that, Cs adsorption experiment was investigated using the sodium alginate-PB compoiste materials.