In a fixing device for an image forming apparatus of the invention, circumferences of both side parts of an elastic layer are made larger than a circumference of a central part, and the expansion of a foamed rubber layer is absorbed by space between the elastic layer and a metal conductive layer. Thereby, the hardness of a heat roller at the time of fixing is made nearly uniform across the entire length in the shaft direction. The contact area of a core member and the elastic layer in the both side parts of the heat roller is made larger, and the stress per unit area generated in the elastic layer due to contact with the pressure roller is reduced.