PURPOSE: To prevent abnormal heat generation at the side part of a fixing heater with such a structure that a temperature detection means is arranged only at the central part thereof.

CONSTITUTION: The temperature detection means 6 is arranged at the central part of the fixing heater 3. Then, the detected temperature of the central part of the heater 3 and the presence and the size of a printing paper are inputted to a neural network. By the neural network, the temperature of the heat generated at the side part of the heater 3 is estimated. By regulating electric power supplied to the heater 3 according to the estimated temperature, abnormal heat generation at the side part of the heater 3 is prevented when the narrow- width printing papers are continuously fixed.