A fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium is disclosed. A first roller incorporates a heat source therein. A second roller is pressed toward the first roller. A part of an endless belt is wound on an outer periphery of the second roller to be circulated. A non-rotatable belt stretcher stretches the endless belt together with the second roller such that the circulated endless belt is slid on a first face extending in a widthwise direction of the endless belt, and such that a tensile force is applied to both widthwise end portions of the endless belt. The circulated endless belt is nipped between the first roller and the second roller to form a first nip portion through which the unfixed toner image is fixed on the recording medium with heat from the heat source. The first face is curved in the widthwise direction.