A filter material for a blood processing filter having different Critical Wetting Surface Tension (CWST) values for one surface A and another surface B, characterized in that the filter material comprises polyester nonwoven fabric; the CWST value for the one surface A, determined as described in the description, is 72 mN/m or more and 110 mN/m or less, and the CWST value, determined as described in the description, for the other surface B is 60 mN/m or more and 110 mN/m or less; the difference in CWST values for surface A and surface B of the filter material is 1 mN/m or more; and the surface A and the surface B have a hydrophilic polymer coating layer, whereby said hydrophilic polymer has a nonionic hydrophilic group and a basic nitrogen-containing functional group, and the amount of basic nitrogen atoms in the polymer coating layer is from 0.2 to 8.0 mass%, wherein the monomer giving rise to said nonionic hydrophilic group in the hydrophilic polymer is selected from 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, vinyl alcohol, (meth) acrylamide, N-vinyl pyrrolidone, alkoxy polyethylene glycol (meth) acrylates, methoxyethylene glycol (meth) acrylate, methoxydiethylene glycol (meth) acrylate, methoxytriethylene glycol (meth) acrylate, and methoxytetraethylene glycol (meth) acrylate; and the monomer giving rise to said basic nitrogen-containing functional group in the hydrophilic polymer is selected from allylamines, dimethylaminoethyl (meth) acrylate, dimethylaminopropyl (meth) acrylate, 3-dimethylamino-2-hydroxyl (meth) acrylate, diethylaminoethyl (meth) acrylate, p-dimethylaminomethylstyrene, p-dimethyl aminoethylstyrene, 2-vinylpyridine, 4-vinylpyridine, 4-vinylimidazole, and quaternary ammonium salt derivatives obtained by subjecting 2-vinylpyridine, 4-vinylpyridine, or 4-vinylimidazole to an alkyl halide. the filter material comprises polyester nonwoven fabric; the CWST value for the one surface A, determined as described in the description, is 72 mN/m or more and 110 mN/m or less, and the CWST value, determined as described in the description, for the other surface B is 60 mN/m or more and 110 mN/m or less; the difference in CWST values for surface A and surface B of the filter material is 1 mN/m or more; and the surface A and the surface B have a hydrophilic polymer coating layer, whereby said hydrophilic polymer has a nonionic hydrophilic group and a basic nitrogen-containing functional group, and the amount of basic nitrogen atoms in the polymer coating layer is from 0.2 to 8.0 mass%, wherein the monomer giving rise to said nonionic hydrophilic group in the hydrophilic polymer is selected from 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, vinyl alcohol, (meth) acrylamide, N-vinyl pyrrolidone, alkoxy polyethylene glycol (meth) acrylates, methoxyethylene glycol (meth) acrylate, methoxydiethylene glycol (meth) acrylate, methoxytriethylene glycol (meth) acrylate, and methoxytetraethylene glycol (meth) acrylate; and the monomer giving rise to said basic nitrogen-containing functional group in the hydrophilic polymer is selected from allylamines, dimethylaminoethyl (meth) acrylate, dimethylaminopropyl (meth) acrylate, 3-dimethylamino-2-hydroxyl (meth) acrylate, diethylaminoethyl (meth) acrylate, p-dimethylaminomethylstyrene, p-dimethyl aminoethylstyrene, 2-vinylpyridine, 4-vinylpyridine, 4-vinylimidazole, and quaternary ammonium salt derivatives obtained by subjecting 2-vinylpyridine, 4-vinylpyridine, or 4-vinylimidazole to an alkyl halide. <EOS>