The temperature "t" sensed at the end in the longitudinal direction of a heater is compared with a threshold Th and, based on the comparison result, the print speed (throughput) is controlled. The threshold Th is switched according to a predetermined condition. For example, whether the number of times the throughput reduction, which switches the print speed to a lower speed, is executed has reached a predetermined number is used as the predetermined condition. Alternatively, whether the small-size-paper passing ratio is higher than a predetermined value is used as the predetermined condition. This prevents grease runoff, provides good slidability between a fixing film and the heater, solves problems of improper paper conveyance, sliding sound generation, and improper fixing, and improves the durability of a fixing device.