To obtain the amorphous iron oxide solving the defects of conventional oral phosphate absorbents, exhibiting an excellent phosphate-absorbing property and capable of being stably used as an absorbent by hydrolyzing an iron salt in the presence of a water-soluble organic compound.

An iron salt such as an iron sulfate is hydrolyzed in the presence of a water-soluble organic compound such as an alcohol or a saccharide, and the obtained gel is sufficiently washed to remove the organic compound and soluble salts. Thus, the amorphous iron oxide in which crystallinity is never recognized by an X-ray analysis is extremely easily obtained. For example, 0.1mol of 2-aminoglucose hydrochloride salt is dissolved in 50ml of oxygen- removed water, mixed with 20ml of 1mol/L aqueous sodium hydroxide solution, stirred for 1hr, mixed with the equimolar amount of iron (II) sulfate, and subsequently stirred for 24hr in an atmosphere of nitrogen gas. After the reaction is finished, the reaction solution is mixed with ethanol, and the obtained precipitates are filtered off, washed and subsequently dried. The obtained iron compound easily binds and absorbs the phosphate similarly as aluminum, but does not have such the toxicity as that of the aluminum. The iron compound does thereby not cause any problem, even if taken in the body.