A blood treatment filter (1A,1B,1C,1D,1E,1F) that treats blood introduced from an inlet and discharges the blood from an outlet, the blood treatment filter (1A,1B,1C,1D,1E,1F) comprising: a filter container that introduces the blood from the inlet into an internal space, and discharges the treated blood from the outlet; a filter material that is housed in the filter container to partition the internal space into an inlet side space and an outlet side space, and filters the passing blood to remove a specific component; an inlet port that forms a blood introduction passage through which the blood flowing from the inlet to the internal space passes; and an outlet port (22A,22B,22C,22D,22E,22F) that forms a blood lead-out passage through which the blood flowing from the internal space to the outlet passes; characterized in that the blood lead-out passage has an outlet tapered portion with an inner diameter increasing from the internal space side toward the outlet side, an outlet anterior chamber (31,61) communicating with the internal space, an outlet connection channel communicating with the outlet and having the outlet tapered portion, and a draw-in opening communicating between the outlet anterior chamber (31,61) and the outlet connection channel, the outlet anterior chamber (31,61) has a pair of side surfaces facing each other with the draw-in opening therebetween, the inner diameter of the draw-in opening being smaller than the separation distance between the pair of side surfaces, an end surface continuing the side surfaces and being the most distant surface from internal space, and a lower surface formed between the side and end surfaces (31c, 31d) and the draw-in opening and being a side path (Ra) for the blood, said outlet anterior chamber (31,61) further comprising a dome-shaped ceiling surface connected to the lower surface by the side surfaces, and the edge of the draw-in opening on the deep side of the chamber is provided such that it does not contact the end surface, the draw-in opening is a starting point portion on the internal space side of the outlet tapered portion, and the starting point portion has an acute-angled cross section if the outlet port (22A,22B,22C,22D,22E,22F) is cut along an imaginary surface including a central axis in a channel direction of the outlet tapered portion. a filter container that introduces the blood from the inlet into an internal space, and discharges the treated blood from the outlet; a filter material that is housed in the filter container to partition the internal space into an inlet side space and an outlet side space, and filters the passing blood to remove a specific component; an inlet port that forms a blood introduction passage through which the blood flowing from the inlet to the internal space passes; and an outlet port (22A,22B,22C,22D,22E,22F) that forms a blood lead-out passage through which the blood flowing from the internal space to the outlet passes; characterized in that the blood lead-out passage has an outlet tapered portion with an inner diameter increasing from the internal space side toward the outlet side, an outlet anterior chamber (31,61) communicating with the internal space, an outlet connection channel communicating with the outlet and having the outlet tapered portion, and a draw-in opening communicating between the outlet anterior chamber (31,61) and the outlet connection channel,