A fixing device fixes a toner image on a recording medium. The fixing device includes a heat source that converts electric power into heat and a fixing member that gives the heat generated by the heat source to the recording medium on which the toner image is formed. The fixing device includes a safety circuit that forcibly interrupts voltage supplied from a power supply to the heat source if the temperature in the device detected by a temperature detection sensor exceeds reference temperature. The fixing device has a malfunction preventing circuit that stops the operation of the safety circuit in order to prevent the voltage supplied to the heat source from being forcibly interrupted when the voltage of the power supply supplied to the heat source is unstable.