In continuous printing operation which repeatedly sets a state in which when a preceding printing material passes through the nip area of a thermal fixing unit, a succeeding printing material has been supplied to an image forming unit and image forming operation has been started, a first temperature indicating an ambient temperature near the thermal fixing unit is measured at an early time point in the continuous printing operation. A second temperature indicating the temperature of the printing material which has passed through the press contact nip portion at a predetermined period of time during the continuous printing operation is measured. The amount of heat to be supplied to the succeeding printing material is controlled on the basis of the measured first and second temperatures.