The purpose of the present invention is to provide: a phosphorus adsorbent which is to be used for adsorbing phosphorous in extracorporeal circulation with little fear of disturbing the environment in vivo ； a phosphorus adsorption column which comprises the phosphorus adsorbent disposed therein； a porous fiber which comprises the phosphorus adsorbent carried therein with little fear of disturbing the environment in vivo ； and a phosphorus adsorption column which comprises the porous fiber disposed therein. To achieve the purpose as described above, the present invention has the following constitution. Namely, the phosphorus adsorbent, which is to be used for adsorbing phosphorus in extracorporeal circulation, is a powder comprising a carbonate of a rare earth metal or an oxide of a Group 4 element and having a solubility in 100 g of water at 20 °C of 10 mg or less. The porous fiber, which comprises the phosphorus adsorbent carried therein, shows a change in pH value of from -1.0 to +1.0 inclusive before and after stirring in physiological saline for 4 hours. The phosphorus adsorption columns respectively comprise the phosphorus adsorbent and the porous fiber each disposed therein.
