Abstract: A filter unit for removing leukocytes from a leukocyte-containing suspension, comprising a container provided with at least one inlet conduit means and at least one outlet conduit means, the container having a main filter packed therein in the form of a non-woven fabric which comprises fibers of an average diameter of from 0.3 .mu.m to less than 3 .mu.m and which has a bulk density of from 0.01 g/cm.sup.3 to 0.7 g/cm.sup.3, and which has an average distance between two adjacent fibers defined by the following equation (1) of from 0.5 .mu.m to 7.0 .mu.m: ##EQU1## wherein y is the average distance between two adjacent fibers in micron; x is the average diameter of fibers in microns; .rho. is the density of the fibers in g/cm.sup.3 ; D is the bulk density of the filter in g/cm.sup.3 ; and .pi. is a circular constant.