An adsorbent capable of adsorbing radioactive antimony, radioactive iodine and radioactive ruthenium, the adsorbent containing cerium(IV) hydroxide in a particle or granular form having a particle size of 250 μm or more and 1200 μm or less; and a treatment method of radioactive waste water containing radioactive antimony, radioactive iodine and radioactive ruthenium, the treatment method comprising passing the radioactive waste water containing radioactive antimony, radioactive iodine and radioactive ruthenium through an adsorption column packed with the adsorbent, to adsorb the radioactive antimony, radioactive iodine and radioactive ruthenium on the adsorbent, wherein the absorbent is packed to a height of 10 cm or more and 300 cm or less of the adsorption column, and wherein the radioactive waste water is passed through the adsorption column at a linear velocity (LV) of 1 m/h or more and 40 m/h or less and a space velocity (SV) of 200 h−1 or less.