To provide a thermal fixing device which can prevent problems of melting and breakage of a member even when a heater having a different resistance temperature coefficient is used for a film heating type thermal fixing device.

The thermal fixing device includes a resistance heating body 23 which generates heat by being fed with a current, a constant current control circuit 32 which feeds a constant current to the resistance heating body 23, a timer which clocks a time of feeding to the resistance heat generating body 23, and a thermistor 28 which detects the temperature of the resistance heating body, wherein when the resistance heat generating body 23 is fed with the constant current from the constant current control circuit 32 to generate heat, a control part 31 determines a temperature rising speed of the resistance heat generating body 23 from detection results of the timer and thermistor 28 and controls the temperature of the resistance heat generating body 23 in accordance with the temperature rising speed.