Provided is a biological fluid-treating filter for treating a biological fluid containing erythrocytes, which has a matrix and a polymer retained by the matrix, the polymer having a zwitterion-containing functional group and a basic nitrogen-containing functional group. By using the filter in which the polymer has the zwitterion-containing functional group and the basic nitrogen-containing functional group, it is possible to treat the biological fluid containing erythrocytes without adversely affecting the erythrocytes.
