A blood filter device comprising: a housing that includes a dome provided with a blood inlet and forming an upper part of the housing, a filter retaining portion positioned below the dome and forming a middle part of the housing, and a bottom portion disposed below the filter retaining portion and provided with a blood outlet; and a filter mounted in a cavity of the filter retaining portion and partitioning a cavity of the housing into a dome side and a bottom portion side, wherein 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 filter retaining portion in parallel, characterized that the blood filter device further comprises: an annular holder disposed to intervene between an inner peripheral surface of the filter retaining portion and an outer peripheral surface of the filter; an annular rib substrate, which is an annular member disposed facing the ridgelines of the plurality of pleats in an outer peripheral region of the filter and is provided with a plurality of ribs inserted respectively between the ridgelines of the plurality of pleats; and a bonding resin that is supplied into outer peripheral portions of the filter including the annular holder and the annular rib substrate, so as to bond the outer peripheral portion of the filter to the filter retaining portion. a housing that includes a dome provided with a blood inlet and forming an upper part of the housing, a filter retaining portion positioned below the dome and forming a middle part of the housing, and a bottom portion disposed below the filter retaining portion and provided with a blood outlet; and a filter mounted in a cavity of the filter retaining portion and partitioning a cavity of the housing into a dome side and a bottom portion side, wherein 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 filter retaining portion in parallel, characterized that the blood filter device further comprises: an annular holder disposed to intervene between an inner peripheral surface of the filter retaining portion and an outer peripheral surface of the filter; an annular rib substrate, which is an annular member disposed facing the ridgelines of the plurality of pleats in an outer peripheral region of the filter and is provided with a plurality of ribs inserted respectively between the ridgelines of the plurality of pleats; and a bonding resin that is supplied into outer peripheral portions of the filter including the annular holder and the annular rib substrate, so as to bond the outer peripheral portion of the filter to the filter retaining portion. an annular holder disposed to intervene between an inner peripheral surface of the filter retaining portion and an outer peripheral surface of the filter; an annular rib substrate, which is an annular member disposed facing the ridgelines of the plurality of pleats in an outer peripheral region of the filter and is provided with a plurality of ribs inserted respectively between the ridgelines of the plurality of pleats; and a bonding resin that is supplied into outer peripheral portions of the filter including the annular holder and the annular rib substrate, so as to bond the outer peripheral portion of the filter to the filter retaining portion.