A fixing device usable with an image forming apparatus having a fixing roller to fix a toner on a recording medium, a heater to heat the fixing roller, and a power supplier to supply an electric power to the heater includes a relay interposed between the power supplier and the heater to supply and interrupt the electric power to the heater, an auxiliary power supplier to supply a driving power to the relay, and a thermoswitch interposed between the relay and the auxiliary power supplier and mounted adjacent to the fixing roller -to be electrically opened when the fixing roller reaches a predetermined overheating temperature. The fixing device is used in an image forming apparatus to reduce manufacturing costs and improve credibility and stability on an overheating prevention operation.