A blood treatment filter provided with a sheet-like filter element and also with an inlet-side flexible container and an outlet-side flexible container which sandwich the filter element and are sealed, the blood treatment filter being configured in such a manner that the blood treatment filter receives untreated blood from the inlet port provided to the inlet-side flexible container and discharges the blood, which has been treated by the filter element, from the outlet port provided to the outlet-side flexible container. The blood treatment filter is characterized in that the blood treatment filter is provided with a first seal section which is formed so as to seal in a band-like shape the inlet-side flexible container and the filter element and to surround the inlet port, and also with an annular second seal section which is formed so as to seal at least the inlet-side flexible container and the outlet-side flexible container and to surround the first seal section at a position further toward the outer edge side than the first seal section, that a valley section corresponding to the first seal section is provided on the outlet side of the filter element, that an outlet opening communicating with the inside of the outlet-side flexible container is formed at the outlet port, and that at least a part of the outlet opening is disposed so as to be superposed on the valley section.La présente invention concerne un filtre destiné à traiter le sang comprenant un élément de filtre de type feuillet ainsi quun récipient flexible du côté de lentrée et un récipient flexible du côté de la sortie, lesquels sont fixés de manière étanche et prennent en sandwich lélément de filtre, le filtre destiné à traiter le sang étant configuré de sorte que le filtre destiné à traiter le sang reçoive du sang non traité par lorifice dentrée situé au niveau du récipient flexible du côté de lentrée et évacue le sang, qui a été traité par le biais de lélément de filtre, par lorifice de sortie situé
