When a fixing device becomes uncontrollable and a large amount of electric power is continuously supplied to the heater, concentration of mechanical stress on a heater is suppressed, whereby shear fracture of the heater is prevented from occurring. A longitudinal region of the heater in which a heat generation resistive member is provided is supported by a heater support surface of a heater holder, and an opposed surface of the heater holder that is opposed to a longitudinal region of the heater in which the heat generation resistive member is not provided is designed not to be in contact with the heater.