There is disclosed a device for fixing a toner image on a recording medium. A heat source is incorporated in a first roller. A heat-resistant belt member is stretched by a second roller and a stretcher and circulated therearound. The belt member is brought into contact with the first roller to form a nip portion therebetween through which the recording medium is passed. At least one of the first roller and the second roller is provided with a cylindrical core member and a rubber layer. The cylindrical core member is formed with a first portion having a first diameter, a second portion having a second diameter which is smaller than the first diameter, and a step portion continuously connecting the first portion and the second portion. The rubber layer is secured to an outer periphery of the core member so as to cover at least the first portion and a part of the step portion.