Blood treatment filter (1) having housing (2) provided with inflow port (4) and outflowport (6) and, placed in the housing (2), filter unit (7) for filtering of bloodcontaining erythrocytes, platelets and leukocytes as blood cell components, characterizedin that the filter unit (7) includes a main filter part capable of substantiallytrapping preselected blood cell components from blood while allowing the remainderof the blood cell components to pass therethrough and a prefilter part disposedupstream of the main filter part and structured to trap substantially no bloodcell components, and that the prefilter part exhibits a microparticle transmissionratio of 70 to 90% as measured by causing a suspension containing microparticlesof 6 μm average particle diameter to flow with a drop distance.