A blood treatment filter 1A comprising: a filter container 3 that introduces blood from an inlet 25 into an internal space R, and discharges treated blood from an outlet 27 a filter material 5 that is housed in the filter container 3 to partition the internal space into an inlet side space R1 and an outlet side space R2, and filters the passing blood to remove a specific component an inlet port 21A that forms an inlet channel 26 through which the blood flowing from the inlet 25 to the internal space R passes an outlet port 22A that forms an outlet channel 28 through which the blood flowing from the internal space R to the outlet 27 passes, wherein the outlet channel 28 has a tapered outlet passage 33 with an inner diameter increasing from the internal space R side toward the outlet 27 side.
