A blood purifier comprises hollow fiber membranes allowing a dialysing fluid or a filtrated fluid to flow outside. The membrane has an effective length of 10 mm or more and 150 mm or less. A maximum value of a permeation flux Jv obtained by dividing a volume of a permeate fluid permeating through the membranes by a membrane area of the hollow fiber membranes and time, is represented as Jvmax. A linear velocity of blood flowing in the membrane is represented as uB. A pressure difference between blood flowing inside the membranes and a fluid flowing outside the membranes is represented as TMP. When a filling blood volume into the membranes per unit membrane area is 20 mL/m2 or more and 35 mL/m2 or less and Jvmax/uB has a value of 0.00015 or more and 0.0006 or less, a TMP change rate is 0.95 or more and 1.05 or less.