A blood filter device includes: a housing (1) that includes a dome portion (2), a filter retaining portion (3), and a bottom portion (4) an inlet (5) provided on a lateral portion of the dome portion so as to allow blood to flow into the dome portion horizontally and along an inner wall of the dome portion an air vent (6) provided at a top of the dome portion a filter (8) for filtering a foreign substance in the blood, which is disposed in the filter retaining portion and an outlet (7) for the blood, which is provided in the bottom portion. The blood filter device is configured so that the blood flows into the dome portion from the inlet, passes through the filter retaining portion, and then flows out from the outlet. The filter is formed of a sheet-like filter member that has been folded so as to have a plurality of pleats (8a) with enveloping surfaces connecting top ends of the respective pleats being flat so that the filter as a whole has a plate-shaped outer shape, and the filter is arranged so as to partition a cavity of the housing into a dome portion side and a bottom portion side. The blood filter device can remove impurities, thrombi, and the like in blood effectively and also can discharge air bubbles remaining in the filter easily
