Image processing apparatus with preheating control for fuser

An image forming apparatus has a fuser that fuses a toner image onto a printing medium by heat and pressure. The fuser temperature is sensed, and a heater in the fuser is controlled to keep the fuser at a constant temperature appropriate for fusing. In addition to constant-temperature control, the fuser is preheated to prevent an anticipated drop in its temperature when the printing medium encounters the fuser. The preheating process is carried out selectively and is omitted when not required, as determined from, for example, the temperature of the fuser when the printing medium enters the image forming unit of the image forming apparatus, the interval between the transport of different sheets of the printing medium, or the average amount of heat generated by the heater during a preceding interval.