PURPOSE: To detect the imperfect contact of the wiring of a thermistor, etc., for measuring the temperature of a heater by judging that abnormality occurs when the temperature difference of the heater is larger than a previously set reference value.

CONSTITUTION: A main body controller 8 is provided with an A/D converter 81 which digitally converts the temperature detecting signal of the thermistor 6, a CPU 82 which outputs an on/off control signal based on the digital value, and a transistor 83 whose operation is controlled according to the on/off control signal from the CPU 82. The contact of both connecting terminal pieces 3 and 7 between a main body frame and a fixing device 4 and the imperfect contact at a detaching part A are detected. The detection of the abnormality is executed at a fixed interval by the CPU 82 after turning on the heater. First, the A/D converted value V1 is inputted, and next a difference between the previous A/D converted value V2 and the value V1 is compared with the previously set reference value for judging the abnormality. When the difference between V1 and V2 is larger, the heater 5 is turned off by judging that the abnormality occurs, then abnormality processing such as output for informing a user of the abnormality is executed.