A blood filter device has a housing that includes: a head portion (2) provided with a blood inlet (5) and forming an upper structure of the housing, a filtration portion (3) positioned below the head portion and forming a middle structure of the housing, and a bottom portion (4) disposed below the filtration portion and provided with a blood outlet (7); and a filter (8) mounted in a cavity of the filtration portion and partitioning a cavity of the housing into a head portion side and a bottom portion side. The filter is formed of a filter sheet folded to have a plurality of pleats, and disposed so that ridgelines of the plurality of pleats traverse respectively the cavity of the filtration portion in parallel. The bottom portion has a conical portion (4a) on the inner bottom face protruding downward to form a conical face. Due to the conical portion of the bottom portion, resistance against blood passing through the housing interior is reduced, and the pressure loss can be suppressed satisfactorily while maintaining sufficient filtration performance of the filter in a small device.