When the surface temperature of a heat roller is less than a toner melting temperature, the heat roller is not driven to rotate in a warm-up operation until the surface temperature of the heat roller reaches a rotatable temperature after a heater is turned ON. However, when the surface temperature drops below the rotatable temperature after the surface temperature once reached the rotatable temperature, the heat roller is immediately driven to rotate after the heater is turned ON, provided the temperature remains above a check temperature.