A fixing device comprising: - a heating member integrally attached to a fixing roller; - a pressurizing member that is brought into pressure contact with the heating member; - a coil wound in a loop shape in a width direction of the heating member, the coil being configured to generate a magnetic flux that inductively heats the heating member; and - a magnetic core disposed near the coil, the magnetic core being configured to lead the magnetic flux to an inductive heat-generating layer formed in the heating member, wherein the magnetic core comprises a plurality of first core parts that are placed so as to enclose the coil in a direction orthogonal to a paper conveyance direction in which a recording medium is conveyed, and further a plurality of second core parts that are placed in hollow parts formed by loops of the coil at both ends in the direction orthogonal to the paper conveyance direction, wherein the plurality of second core parts have a lower Curie temperature than the plurality of first core parts and have a smaller thermal capacity than the plurality of first core parts, and wherein the fixing device further comprises a controller adapted to control the fixing device in such a way that when, after a fixing process is performed on a first recording medium having a smaller width than a recording medium with a maximum width up to which fixing is possible and a surface temperature in paper non-feeding areas is raised above an upper fixing-ready temperature limit, fixing process is performed on a second recording medium having a larger width than the first recording medium, the second recording medium is inserted into a nip part formed by the heating member and the pressurizing member after the surface temperature in paper non-feeding areas drops to or is below the upper fixing-ready temperature limit. - a heating member integrally attached to a fixing roller; - a pressurizing member that is brought into pressure contact with the heating member; - a coil wound in a loop shape in a width direction of the heating member, the coil being configured to generate a magnetic flux that inductively heats the heating member; and - a magnetic core disposed near the coil, the magnetic core being configured to lead the magnetic flux to an inductive heat-generating layer formed in the heating member, wherein the magnetic core comprises a plurality of first core parts that are placed so as to enclose the coil in a direction orthogonal to a paper conveyance direction in which a recording medium is conveyed, and further a plurality of second core parts that are placed in hollow parts formed by loops of the coil at both ends in the direction orthogonal to the paper conveyance direction, wherein the plurality of second core parts have a lower Curie temperature than the plurality of first core parts and have a smaller thermal capacity than the plurality of first core parts, and wherein the fixing device further comprises a controller adapted to control the fixing device in such a way that when, after a fixing process is performed on a first recording medium having a smaller width than a recording medium with a maximum width up to which fixing is possible and a surface temperature in paper non-feeding areas is raised above an upper fixing-ready temperature limit, fixing process is performed on a second recording medium having a larger width than the first recording medium, the second recording medium is inserted into a nip part formed by the heating member and the pressurizing member after the surface temperature in paper non-feeding areas drops to or is below the upper fixing-ready temperature limit.