An image fixing device of an image forming apparatus includes a pressure roller and a heating roller to rotate as if it is in contact with the pressure roller, with a paper fed therebetween, to fix a developed image to the paper by applying heat generated by a heating member installed inside therein. The heating roller is a heat pipe of a closed tube type. The heat pipe of the closed tube type has a fluid poured inside of a closed space thereof. The heating member is an induction heating body to generate a magnetic field to the heating roller. The heat generated by induction heating is applied to the heating roller including the heat pipe to keep a temperature around the surface thereof at a predetermined uniform level.