An image forming device for forming a toner image on a recording paper based on a print job including image information, the image forming device comprising: a heated body for fusing toner to the recording paper by heat; a heater for heating the heated body; a light receiving body located so as not to be in contact with the heated body; a first temperature detector (31, 32, 70, 90) for detecting, based on an amount of infrared rays incident upon the light receiving body from the heated body, a first temperature equivalent to a surface temperature of the heated body; a heat receiving body located so as not to be in contact with the heated body; a second temperature detector (70, 90) for detecting, based on a temperature of the heat receiving body heated by the heated body, a second temperature equivalent to the surface temperature of the heated body; a temperature controller for selectively executing either first temperature control for controlling an output of the heater so that the first temperature follows a predetermined target temperature, or second temperature control for controlling the output of the heater so that the second temperature follows the target temperature; a timer (91, 92) for measuring an elapsed time during which the second temperature control is continuously executed; and a warning device for providing notification of an abnormality in the first temperature detector (31, 32, 70, 90), wherein the temperature controller is arranged to perform the first temperature control when a detected temperature difference (ΔT) between the first temperature and the second temperature is less than a predetermined temperature difference, and to perform the second temperature control when the detected temperature difference (ΔT) is equal to or greater than the predetermined temperature difference, and to activate the warning device when the elapsed time has become equal to or greater than a predetermined time, characterised in that the temperature controller is arranged to set the predetermined time to the sum of a first predetermined time and a second predetermined time when the temperature controller starts second temperature control in a state in which the predetermined target temperature (TR; TP; TS) has never been reached by the first temperature control after activation of the image forming device, and/or when the temperature controller starts the second temperature control after occurrence of a print job, wherein the first predetermined time is a time required for condensation on the light receiving body to be eliminated by continuation of heating, and the temperature controller is arranged to set the predetermined time to the second predetermined time when the temperature controller starts the second temperature control in a state in which the predetermined target temperature (TR; TP; TS) has been reached by the first temperature control and then no print job has occurred. a heated body for fusing toner to the recording paper by heat; a heater for heating the heated body; a light receiving body located so as not to be in contact with the heated body; a first temperature detector (31, 32, 70, 90) for detecting, based on an amount of infrared rays incident upon the light receiving body from the heated body, a first temperature equivalent to a surface temperature of the heated body; a heat receiving body located so as not to be in contact with the heated body; a second temperature detector (70, 90) for detecting, based on a temperature of the heat receiving body heated by the heated body, a second temperature equivalent to the surface temperature of the heated body; a temperature controller for selectively executing either first temperature control for controlling an output of the heater so that the first temperature follows a predetermined target temperature, or second temperature control for controlling the output of the heater so that the second temperature follows the target temperature; a timer (91, 92) for measuring an elapsed time during which the second temperature control is continuously executed; and a warning device for providing notification of an abnormality in the first temperature detector (31, 32, 70, 90), wherein the temperature controller is arranged to perform the first temperature control when a detected temperature difference (ΔT) between the first temperature and the second temperature is less than a predetermined temperature difference, and to perform the second temperature control when the detected temperature difference (ΔT) is equal to or greater than the predetermined temperature difference, and to activate the warning device when the elapsed time has become equal to or greater than a predetermined time, characterised in that the temperature controller is arranged to set the predetermined time to the sum of a first predetermined time and a second predetermined time when the temperature controller starts second temperature control in a state in which the predetermined target temperature (TR; TP; TS) has never been reached by the first temperature control after activation of the image forming device, and/or when the temperature controller starts the second temperature control after occurrence of a print job, wherein the first predetermined time is a time required for condensation on the light receiving body to be eliminated by continuation of heating, and the temperature controller is arranged to set the predetermined time to the second predetermined time when the temperature controller starts the second temperature control in a state in which the predetermined target temperature (TR; TP; TS) has been reached by the first temperature control and then no print job has occurred.