Disclosed is a thermal fixing device using at least two heaters for respectively heating center and end portions of a fixing member, in which after the driving of the two heaters is stopped, thermal fixing can be re-started quickly at proper temperatures for both the center and end portions. First, in S100, a judgment is made as to whether the center portion temperature of the thermal roller exceeds 100.degree. C. or not. When it exceeds 100.degree. C., the procedure advances to S110, where the center heater is turned ON two seconds after turning ON the side heater. By thus turning ON the center heater two seconds after the turning ON of the side heater, it is possible to reduce the temperature difference between the two heaters to approximately 8 to 13.degree. C. at the time point when the center heater is turned ON. In this way, the end portions of the thermal roller are heated prior to the center portion thereof, so that, as compared with the case in which the center heater and the side heater are turned ON simultaneously, the temperature difference between these portions is smaller.