A standard resistor having a known resistance value is connected to one transistor in a drive IC. The drive IC has a plurality of transistors each controlling the on/off of a heating element. The saturation voltage of each transistor is generally the same. One of a plurality of transistors is turned on, and a discharge time required for a capacitor to discharge via a standard resistor or via each heating element and lower its voltage to a predetermined voltage is measured. The resistance value of each heating element is determined from a ratio of a discharge time via each heating element to a discharge time via the standard resistor and the resistance value of the standard resistor. A difference between the standard resistance value and a resistance value of each heating element generates a bias heat energy error during bias heating and an image heat energy error during image heating. In accordance with these heat energy errors, bias data for generating a heat energy immediately before coloring is corrected. Each heating element is driven by using the corrected bias data, and the heat energy errors are corrected at the time of bias heading. At the time of image heating, each heating element is driven by using image data without considering the resistance value error.
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