A fixing device, comprising: a rotatable endless belt fixing member; a pressure member located at an outer circumferential side of the fixing member, configured to press against the fixing member; a nip forming member located at an inner circumferential side of the fixing member, configured to contact the pressure member through the fixing member to form a nip; a reinforcing member fixed at a bore of the fixing member, configured to support the nip forming member from the opposite side of the nip; a heating member configured to directly or indirectly heat a predetermined area of the fixing member at an upstream side of the nip; and a flange member comprising a cylinder inserted in an inner circumference of the fixing member at an edge of the fixing member in an axial direction thereof and configured to rotatably hold an edge vicinity of the fixing member directly or indirectly with the outer circumferential surface of the cylinder, and a flange fixed on a frame of the fixing device, wherein the cylinder of the flange member comprises a notch (28a1) in the circumference configured to contain the nip forming member, and further comprises an arc-shaped outer circumferential cross-section at an area corresponding to an area where the fixing member is heated by the heating member, having a predetermined radius equivalent to a radius of the fixing member, and the arc-shaped outer circumferential cross-section has a center located at an upstream side in a recording medium feeding direction relative to a center line of the nip forming member in the recording medium feeding direction, characterized in that the nip forming member includes a flat surface and a projection from an upstream side in a recording medium feeding direction at a surface of the pressure member side. a rotatable endless belt fixing member; a pressure member located at an outer circumferential side of the fixing member, configured to press against the fixing member; a nip forming member located at an inner circumferential side of the fixing member, configured to contact the pressure member through the fixing member to form a nip; a reinforcing member fixed at a bore of the fixing member, configured to support the nip forming member from the opposite side of the nip; a heating member configured to directly or indirectly heat a predetermined area of the fixing member at an upstream side of the nip; and a flange member comprising a cylinder inserted in an inner circumference of the fixing member at an edge of the fixing member in an axial direction thereof and configured to rotatably hold an edge vicinity of the fixing member directly or indirectly with the outer circumferential surface of the cylinder, and a flange fixed on a frame of the fixing device, wherein the cylinder of the flange member comprises a notch (28a1) in the circumference configured to contain the nip forming member, and further comprises an arc-shaped outer circumferential cross-section at an area corresponding to an area where the fixing member is heated by the heating member, having a predetermined radius equivalent to a radius of the fixing member, and the arc-shaped outer circumferential cross-section has a center located at an upstream side in a recording medium feeding direction relative to a center line of the nip forming member in the recording medium feeding direction, characterized in that the nip forming member includes a flat surface and a projection from an upstream side in a recording medium feeding direction at a surface of the pressure member side.