A fuel injection device comprising: a valve body that comes into contact with a valve seat, to close a fuel passage, and moves away from the valve seat, to open the fuel passage; a mover that performs valve opening and closing operations in collaboration with the valve body, wherein the valve body and the mover are relative-displaceably formed separate members, and wherein the valve body has a regulator that regulates a relative displacement of the mover with respect to the valve body in a valve opening direction; and the fuel injection device further comprising a magnetic core that is excited by energization of a coil and that generates a magnetic force to drive the mover, wherein the magnetic core has a hole that is opened in an end surface (206, 604) opposite to the mover and formed in a direction along a valve stem, and wherein the hole has an inner diameter enlarged part (201, 701, 702, 801,180, 181, 550), In which an inner diameter is gradually enlarged from a back side of the hole toward the end surface (206, 604), in an opening to the end surface (206, 604), further wherein an interval between a surface where the inner diameter enlarged part (201, 701, 702, 801, 180, 181, 550) is formed and the mover in the direction along the valve stem is Increased as the inner diameter of the inner diameter enlarged part (201, 701, 702, 801, 180, 181, 550) becomes smaller, wherein an attracting surface of the magnetic core is the mover side end surface of the magnetic core, and in the hole formed in the magnetic core, a part where an inner diameter of the hole is minimum is formed in the back of the position of the regulator. a valve body that comes into contact with a valve seat, to close a fuel passage, and moves away from the valve seat, to open the fuel passage; a mover that performs valve opening and closing operations in collaboration with the valve body, wherein the valve body and the mover are relative-displaceably formed separate members, and wherein the valve body has a regulator that regulates a relative displacement of the mover with respect to the valve body in a valve opening direction; and the fuel injection device further comprising a magnetic core that is excited by energization of a coil and that generates a magnetic force to drive the mover, wherein the magnetic core has a hole that is opened in an end surface (206, 604) opposite to the mover and formed in a direction along a valve stem, and wherein the hole has an inner diameter enlarged part (201, 701, 702, 801,180, 181, 550), In which an inner diameter is gradually enlarged from a back side of the hole toward the end surface (206, 604), in an opening to the end surface (206, 604), further wherein an interval between a surface where the inner diameter enlarged part (201, 701, 702, 801, 180, 181, 550) is formed and the mover in the direction along the valve stem is Increased as the inner diameter of the inner diameter enlarged part (201, 701, 702, 801, 180, 181, 550) becomes smaller, wherein an attracting surface of the magnetic core is the mover side end surface of the magnetic core, and in the hole formed in the magnetic core, a part where an inner diameter of the hole is minimum is formed in the back of the position of the regulator.