A blood purifying apparatus having excellent purification performance (elimination of low molecular proteins, etc.) and being contaminated with little endotoxins flowing from the dialyzate side is described. Attention was paid to the solute permeability coefficient &agr; and water permeability performance Lp of blood purifying apparatus and the relation between &agr; and Lp has been examined. As a result, a blood purifying apparatus having excellent purification performance and substantially being free from invasion of endotoxins was obtained by regulating the value &agr;/Lp to 6×10−7 or above, or regulating &agr; within a range of 8×105 to 1.5×10−3, and the value &agr;×Lp to 2.4×10−2 or less. Also, a blood purifying apparatus having excellent purification performance and substantially being free from invasion of endotoxins was obtained by regulating the invasion ratio which is obtained by the polymer invasion test to 10% or less.