PROBLEM TO BE SOLVED: To provide an oral phosphorus adsorbent which can adsorb phosphorus in a body by a lanthanum compound, and can also effectively inhibit adsorption of lanthanum derived from the lanthanum compound.SOLUTION: When administering a lanthanum compound as an oral phosphorus adsorbent, combined use of the alkali-metal salt of alginic acid with an average molecular weight of 1,100,000 or less can effectively inhibit the absorption of lanthanum to a living organism, while effectively exhibiting the phosphorus adsorption capability of the lanthanum compound.SELECTED DRAWING: None