A thermal fixing device is equipped with a plurality of heaters for heating a fixing member at different axial positions thereof for proper temperature control. When the difference between the center temperature T_A and the end temperature T_B exceeds a proper temperature difference range (15 C.), of the center temperature T_A and the end temperature T_B, the one nearer to the average temperature T_m of the fixing upper limit temperature T_h and the fixing lower limit temperature T₁. is selected. The control value for the farther halogen lamp is changed so as to approach the selected temperature. When the temperature difference is 10 C. or less, the control value is further changed. With this process, it is possible to set both the temperatures and the portion near the center of the thermal roller to temperatures within the proper temperature range.