PURPOSE: To prevent copying paper from curling and to utilize a composite copying function effectively by holding the surface temperature of a fixing roller below the temperature of single-original copying in a fixing process of starting fixation in composite copying mode.

CONSTITUTION: A fixing device consists of the fixing roller 1 and a pressure roller 2 pressed against the fixing roller 1 and the fixing roller 1 is held at such a temperature that toner is fixed on a copying form by a heater lamp. A thermistor 3 arranged nearby the fixing roller 1 varies in resistance value with its detected temperature. The surface temperature of the fixing roller is set to the control temperature almost in the middle between control temperature in said normal copy mode and the lower limit temperature of fixation and the fixing performance of the toner does not deteriorate even at the middle control temperature. Consequently, the copying form is prevented from curling in the fixing process of starting copying operation and copy images are formed securely in the 2nd and succeeding copying operations.