A fixing device includes a fuser roller having a built-in heat source therein and an elastic member layered on the outer surface thereof; a pressure roller to be pressed against the fuser roller; a heat-resistant belt which is wound around the outer periphery of the pressure roller and is sandwiched between the pressure roller and the fuser roller so as to travel; and a belt tensioning member for tensioning the heat-resistant belt. The belt tensioning member is arranged on the upstream side in the traveling direction of the heat-resistant belt relative to the pressed portion of the fuser roller and the pressure roller and near the fuser roller beyond the tangent to the pressed portion to wrap the heat-resistant belt around the outer periphery of the fuser roller to form a nip.