An image forming apparatus, including a heat roller in which a heating source is embedded, a press roller installed close to the heat roller, the press roller contacting the heat roller with a predetermined pressurizing force to form a fixing nip, and at least two overheat prevention devices installed near the heat roller to cut off a power supply to the heating source upon an overheating of the heat roller. One of the overheat prevention devices is installed at portion of the heat roller corresponding to a location within a width of a recording medium passing through the fixing nip and another is installed at a site near an end of the heat roller corresponding to a location outside the width of the recording medium.