Disclosed is a novel hydrous composite oxide of cerium (IV) and phosphorus characterized by the specified chemical composition and the unique X-ray diffractometric pattern, which is useful as an adsorbent or immobilizing agent of strontium ions in an aqueous solution such as radioactive wastes from atomic power plants. The composite oxide prepared by the precipitation reaction of a Ce (IV) salt in an aqueous solution and phosphoric acid can be imparted with an increased adsorption capacity for strontium ions when the precipitates are heat-treated under hydrothermal conditions at up to 300.degree. C. Immobilization of strontium ions adsorbed on the adsorbent can be more complete when the adsorbent bearing strontium ions is dried and subjected to a heat treatment at a temperature of 200.degree. C. or higher.