A blood treatment filter comprising: a sheet-shaped filter element; an inlet-side flexible container and an outlet-side flexible container that are sealed, with the filter element being interposed therebetween; an inlet port provided toward the inlet-side flexible container side with respect to the filter element; and a tubular outlet port provided toward the outlet-side flexible container side with respect to the filter element, wherein blood introduced from the inlet port and treated by the filter element is discharged from the outlet port, wherein the filter further comprises: an outlet-side frame sheet disposed between the filter element and the outlet-side flexible container; a first seal portion provided by sealing at least the filter element and the outlet-side frame sheet in a belt-shaped manner; and an annular second seal portion provided by sealing at least the inlet-side flexible container, the outlet-side frame sheet, the outlet port, and the outlet-side flexible container in this order to surround the first seal portion, wherein on an outlet side of the filter element, the filter element is compressed in the first seal portion to form a valley portion, the valley portion comprises an internal slope portion and an external slope portion on the opposite sides of a bottom portion, the outlet port includes a protruding portion that protrudes to the inside of the outlet-side flexible container, the protruding portion is provided with an opening at least a part of which overlaps with the valley portion formed at the first seal portion and which can communicate with a gap region formed by the valley portion, and wherein the distal end of protruding portion is positioned just between the first seal portion and the outlet-side container. a sheet-shaped filter element; an inlet-side flexible container and an outlet-side flexible container that are sealed, with the filter element being interposed therebetween; an inlet port provided toward the inlet-side flexible container side with respect to the filter element; and a tubular outlet port provided toward the outlet-side flexible container side with respect to the filter element, wherein blood introduced from the inlet port and treated by the filter element is discharged from the outlet port, wherein the filter further comprises: an outlet-side frame sheet disposed between the filter element and the outlet-side flexible container; a first seal portion provided by sealing at least the filter element and the outlet-side frame sheet in a belt-shaped manner; and an annular second seal portion provided by sealing at least the inlet-side flexible container, the outlet-side frame sheet, the outlet port, and the outlet-side flexible container in this order to surround the first seal portion, wherein on an outlet side of the filter element, the filter element is compressed in the first seal portion to form a valley portion, the valley portion comprises an internal slope portion and an external slope portion on the opposite sides of a bottom portion, the outlet port includes a protruding portion that protrudes to the inside of the outlet-side flexible container, the protruding portion is provided with an opening at least a part of which overlaps with the valley portion formed at the first seal portion and which can communicate with a gap region formed by the valley portion, and wherein the distal end of protruding portion is positioned just between the first seal portion and the outlet-side container. an outlet-side frame sheet disposed between the filter element and the outlet-side flexible container; a first seal portion provided by sealing at least the filter element and the outlet-side frame sheet in a belt-shaped manner; and an annular second seal portion provided by sealing at least the inlet-side flexible container, the outlet-side frame sheet, the outlet port, and the outlet-side flexible container in this order to surround the first seal portion, wherein on an outlet side of the filter element, the filter element is compressed in the first seal portion to form a valley portion, the valley portion comprises an internal slope portion and an external slope portion on the opposite sides of a bottom portion, the outlet port includes a protruding portion that protrudes to the inside of the outlet-side flexible container, the protruding portion is provided with an opening at least a part of which overlaps with the valley portion formed at the first seal portion and which can communicate with a gap region formed by the valley portion, and wherein the distal end of protruding portion is positioned just between the first seal portion and the outlet-side container.