The present invention relates to a highly water-permeable hollow fiber membrane type blood purifier which comprises hydrophobic polymer hollow fiber membranes each containing a hydrophilic polymer, wherein the hollow fiber membrane has a hydrophilic polymer content of 25 to 50 mass % and a ratio of hole areas of 8 to 25% at its outer surface, and has a thickness non-uniformity degree of 0.6 or more, a thickness of 10 to 60 μm and a burst pressure of 0.5 to 2 MPa, and which is characterized in that the blood purifier has a water permeability of 150 to 2,000 ml/m2/hr/mmHg, and in that said blood purifier is exposed to radioactive rays on conditions that the oxygen concentration of an ambient atmosphere around the hollow fiber membranes is from 0.001% inclusive to 0.1% inclusive, and that the moisture content of the hollow fiber membrane to its weight is from 0.2 mass % inclusive to 7 mass % inclusive. The present invention also relates to a process for manufacturing the same-blood purifier.