To provide a regenerated cellulose-based membrane for purifying blood which is excellent in performance of removing low molecular weight protein represented by β2-microglobulin, particularly has a function of absorbing and removing the protein and the same function of removing it as that of a conventional membrane, and to provide a method for preparing the membrane and a blood dialyzer equipped with the cellulose-based membrane for purifying blood.

The membrane is made of a cellulose and contains a reactive sizing agent. Further, the agent is one kind agent selected from among an alkylketene dimer, an alkenylketene dimer, an alkylsuccinic anhydride or alkenylsuccinic anhydride.