A fixing unit of an image forming apparatus includes a magnetism adjusting unit switching the position of a shielding member between a shielding position where the shielding member is positioned inside the sheet-conveyed region to shield the pass of the magnetism and a retracted position where the shielding member is positioned outside the sheet-conveyed region to permit the pass of the magnetism. The shielding member is provided in a plural number in the axial direction of the movable core. The shielding members have a different length in the axial direction and a different width in a circumferential direction of the movable core, the length and the width corresponding to a plurality of sizes of the sheet in the width direction of the sheet. The magnetism adjusting unit switches each of the shielding members between the shielding position and the retracted position in accordance with the width-direction size of the sheet.