A fixing device is provided in image forming apparatuses such as a printer and the like and performs a fixing by heating to a recording sheet on which a toner image is transferred. The fixing device includes: a heating source adapted for performing fixing of a toner image onto a recording medium by heat; a fixing roller including a thin-wall cylinder rotatable about an axis thereof and carrying the heating source therein, the recording medium being placed on a peripheral surface of the cylinder; an annular heat-insulating member coaxially mounted on an outside surface of the fixing roller; and a C-shaped stopper attached to the peripheral surface of the fixing roller for preventing the annular heat-insulating member from moving out of the fixing roller, the C-shaped stopper including at least three engaging portions.