A fixing device for thermally fixing an unfixed image on a recording sheet, comprising: a heating rotor including a cylindrical resistance heating layer, and fixing an unfixed image on a recording sheet by thermal fusion bonding using heat generated by the resistance heating layer supplied with power; a power supply unit supplying power to the resistance heating layer; an elongated heat sensitive resistor extending along an entire length of an axis of the resistance heating layer, located to face a portion of a circumferential surface of the resistance heating layer, and exhibiting a change in resistance according to a temperature of the portion of the resistance heating layer; and an abnormal heat determination unit determining whether the temperature of the portion has reached an abnormal temperature by detecting the change in resistance of the heat sensitive resistor, the abnormal temperature possibly causing damage to the resistance heating layer.