A pressing/sintering method was used for the stable solidification of spent cesium adsorbent by adding natural allophane and mordenite. The stable Cs4Al4Si20O48 crystalline phase was recrystallized by the reaction of Cs2O, Al2O3, and SiO2 above 1000 degrees C, and the Cs immobilization ratio of approximately 100% was maintained. The solidified products exhibited excellent leaching resistance in distilled water and 0.1 M NaCl. The leaching rate of Cs at 90 degrees C was in the order of H2O approximate to 0.1 M NaCl << 0.1 M HNO3 < 0.1 M NaOH.