An image forming unit of a copier is provided with an image fixing unit (fixing device) adopting an induction heating method. A heating roller of this image fixing unit includes a heat producing layer made of a temperature-sensitive magnetic material whose permeability changes with temperature, and a supporting layer disposed adjacent to the heat producing layer and made of a nonmagnetic and electrically conductive material. The heat producing layer has a width substantially equal to a passing area (about 294 mm) for sheets along a direction of an axis of rotation of the heating roller and smaller than a width (about 310 mm) of an induction heating coil.