The invention provides a blood purifier which shows a decreased amount of hydrogen peroxide extracted from its selectively permeable separation membranes, and thus is highly reliable in its safety in use for hemecatharysis. The blood purifier comprises selectively permeable separation membranes as a main component and is characterized in that the amount of hydrogen peroxide which is extracted from the selectively permeable separation membrane removed from the blood purifier after 3 months or longer has passed since the sterilization of the blood purifier by exposure to a radioactive ray and/or an electron ray is not larger than 10 ppm.