Fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium

A fixing device for fixing an unfixed toner image formed on a sheet-shaped recording medium, the fixing device comprising:

• a first roller, incorporating a heat source therein;
• a second roller, being pressed toward the first roller;
• an endless belt, a part of which is wound on an outer periphery of the second roller to be circulated;
• a non-rotatable belt stretcher, which 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, and
• a wall member, having a second face adjacent to at least one of the both widthwise end portions of the endless belt in order to restrict a wobbling motion of the circulated endless belt, wherein
• 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;
• and
• the first face of the belt stretcher is curved in the widthwise direction of the endless belt.