A fixing device characterized by comprising: an endless belt looped into a generally cylindrical configuration extending in an axial direction thereof; a fuser pad extending in the axial direction inside the loop of endless belt; a pressure member extending in the axial direction with the belt interposed between the fuser pad and the pressure member, the pressure member being pressed against the fuser pad through the fuser belt to form a fixing nip through which a recording medium travels from upstream to downstream in a tangential direction substantially perpendicular to the axial direction; a generally cylindrical, tubular belt holder extending in the axial direction inside the loop of endless belt to retain the belt in shape along an outer circumference thereof, the belt holder accommodating the fuser pad in a longitudinal side slot defined on one side thereof by a pair of opposed first and second walls (22a1, 22a2) extending inward from the holder circumference, the former being upstream and the latter downstream of the fixing nip in the tangential direction, wherein a third wall (22a3) extends between the first wall (22a1) and the second wall (22a2) in the tangential direction; and a reinforcing member disposed inside the belt holder to reinforce the belt holder around the side slot, the reinforcing member including: a first flange (23a1); and a second flange (23a2), characterized in that the first flange (23a1) extends along the first wall (22a1) inside the belt holder and the second flange (23a2) extends along the second wall (22a2) inside the belt holder, wherein the first flange and the second flange (23a2) are connected together by a web (23a3) extending in the tangential direction along the third wall (22a3) of the side slot in order to form a clamping frame which is configured to clamp together the first wall (22a1) and the second wall (22a2) to retain the belt holder in shape. an endless belt looped into a generally cylindrical configuration extending in an axial direction thereof; a fuser pad extending in the axial direction inside the loop of endless belt; a pressure member extending in the axial direction with the belt interposed between the fuser pad and the pressure member, the pressure member being pressed against the fuser pad through the fuser belt to form a fixing nip through which a recording medium travels from upstream to downstream in a tangential direction substantially perpendicular to the axial direction; a generally cylindrical, tubular belt holder extending in the axial direction inside the loop of endless belt to retain the belt in shape along an outer circumference thereof, the belt holder accommodating the fuser pad in a longitudinal side slot defined on one side thereof by a pair of opposed first and second walls (22a1, 22a2) extending inward from the holder circumference, the former being upstream and the latter downstream of the fixing nip in the tangential direction, wherein a third wall (22a3) extends between the first wall (22a1) and the second wall (22a2) in the tangential direction; and a reinforcing member disposed inside the belt holder to reinforce the belt holder around the side slot, the reinforcing member including: a first flange (23a1); and a second flange (23a2), characterized in that the first flange (23a1) extends along the first wall (22a1) inside the belt holder and the second flange (23a2) extends along the second wall (22a2) inside the belt holder, wherein the first flange and the second flange (23a2) are connected together by a web (23a3) extending in the tangential direction along the third wall (22a3) of the side slot in order to form a clamping frame which is configured to clamp together the first wall (22a1) and the second wall (22a2) to retain the belt holder in shape. a first flange (23a1); and a second flange (23a2), characterized in that the first flange (23a1) extends along the first wall (22a1) inside the belt holder and the second flange (23a2) extends along the second wall (22a2) inside the belt holder, wherein the first flange and the second flange (23a2) are connected together by a web (23a3) extending in the tangential direction along the third wall (22a3) of the side slot in order to form a clamping frame which is configured to clamp together the first wall (22a1) and the second wall (22a2) to retain the belt holder in shape.