Provided are an adsorbent capable of removing radioactive water liquid including iodine compounds and/or antimony by means of a water passing treatment, and a method and an apparatus for treating radioactive waste liquid by using the adsorbent. The adsorbent includes a polymer resin and 10 parts by weight or more of a hydrous hydroxide of a rare earth element based on 100 parts by weight of the polymer resin, in which the hydrous hydroxide of the rare earth element has a water content of 1 part by weight to 30 parts by weight based on 100 parts by weight of a dry product thereof, and adsorbs iodine compounds and/or antimony.