PURPOSE: To control fixation temperature precisely by bringing a sensor into contact with a heat fixing roller without considering the insulation distance between the heat fixing roller and a control part.

CONSTITUTION: A signal is transmitted from a temperature detection part 3 to the control part 4 through a photocoupler 6 and a signal is transmitted from the control part 4 to a heating power supply part 5 through a pulse transformer 7. Further, the primary side consisting of the sensor 1, heater 2, temperature detection part 3, and heating power supply part 5 is insulated completely from the secondary side of the control part 4. Consequently, the sensor 1 is put closer to the heater without considering the insulation distance between the control part 4 and heater 2.