A fixing device for fixing a toner image on a recording medium, comprising: an endless belt-shaped fixing member to rotate in a predetermined direction, formed in a loop; a pressing member disposed in contact with an outer circumferential surface of the fixing member; a contact member provided inside the loop formed by the fixing member and pressed against the pressing member via the fixing member to form a nip between the pressing member and the fixing member through which the recording medium bearing the toner image passes, characterized by a laminated heater contacting or disposed close to an inner circumferential face of the fixing member to heat the fixing member, connected to an external power source, and comprising a heat generation sheet comprising a heat-resistant resin in which conductive particles to receive electricity from the external power source and generate heat are unevenly dispersed throughout the heat-resistant resin to have a dispersal gradient of increasing particle dispersion density from an inner face toward an outer face of the heat generation sheet; and a generally cylindrical heater support to support the laminated heater along the inner circumferential face of the fixing member, and to which the contact member is fitted, the heater support sustaining the fixing member in its looped form, wherein the laminated heater is attached to an outer circumferential face of the heater support, wherein a retainer is configured to support the laminated heater between the outer circumferential face of the heater support and the retainer and to fix the laminated heater on the outer circumferential face of the heater support, wherein the retainer is removably attached to the heater support. an endless belt-shaped fixing member to rotate in a predetermined direction, formed in a loop; a pressing member disposed in contact with an outer circumferential surface of the fixing member; a contact member provided inside the loop formed by the fixing member and pressed against the pressing member via the fixing member to form a nip between the pressing member and the fixing member through which the recording medium bearing the toner image passes, characterized by a laminated heater contacting or disposed close to an inner circumferential face of the fixing member to heat the fixing member, connected to an external power source, and comprising a heat generation sheet comprising a heat-resistant resin in which conductive particles to receive electricity from the external power source and generate heat are unevenly dispersed throughout the heat-resistant resin to have a dispersal gradient of increasing particle dispersion density from an inner face toward an outer face of the heat generation sheet; and a generally cylindrical heater support to support the laminated heater along the inner circumferential face of the fixing member, and to which the contact member is fitted, the heater support sustaining the fixing member in its looped form, wherein the laminated heater is attached to an outer circumferential face of the heater support, wherein a retainer is configured to support the laminated heater between the outer circumferential face of the heater support and the retainer and to fix the laminated heater on the outer circumferential face of the heater support, wherein the retainer is removably attached to the heater support.