PURPOSE: To prevent the fault of an image caused by the change of a form feeding quantity based on the thickness of form and the temperature change of a fixing roll, to provide the high speed operation of a copying machine and to miniaturize the copying machine.

CONSTITUTION: Fixing rolls 1 and 2 are rotated by a servo motor 3, and a heater 6 and the speed of the roll are controlled based on temperature information from a sensor 5 for controlling the temperature. When the fixing roll pair is a fixed type one whose axle-to-axle distance is not changed, nip width is changed when the diameter of the roll is changed in accordance with the temperature of the roll. Therefore, in the case of setting the speed of the roll constant, the paper feeding quantity is changed. There is a fixed relation between the paper feeding quantity and output frequency to the servo motor 3. Consequently, by previously storing the relation between the output frequency and the temperature of the roll, setting the output frequency obtained based on the detected value of the temperature of the roll, and controlling the speed of the roll, the relation of the form feeding quantity in the fixing device to the form feeding quantity in a transfer part is always kept stable, and the fault of the image caused by the change of the form feeding quantity in the fixing device is eliminated.