It is intended to provide a polymer which is scarcely eluted, has a high biocompatibility and is useful in a filter for selectively eliminating leucocytes. It is also intended to provide a filter for selectively eliminating leucocytes, a filtration apparatus for selectively eliminating leucocytes and a system for selectively eliminating leucocytes, each having the above-described polymer. The above objects can be achieved by providing a polymer which comprises from 8% by mol to 45% by mol of a unit originating in a polymerizable monomer having a polyalkylene oxide chain, from 30% by mol to 90% by mol of a unit originating in a polymerizable monomer having a hydrophobic group, and from 2% by mol to 50% by mol of a unit originating in polymerizable monomer having a hydroxyl group.