A fixing device comprising: a fixing member; a pressure member pressed against the fixing member; a plurality of halogen lamp heaters (186 to 188) configured to heat the fixing member; a temperature detecting unit configured to detect a temperature of the fixing member; and a control unit configured to perform half-wave control on at least one first halogen lamp heater of the halogen lamp heaters (186 to 188) by calculating an output Duty in accordance with the temperature detected by the temperature detecting unit and performing energization or de-energization for each half-wave of an AC waveform in accordance with the output Duty, and perform switch on/off control on a second halogen lamp heater (187 and 188) of the halogen lamp heaters (186 to 188), the second halogen lamp heater (187 and 188) being other than the first halogen lamp heater, characterized in that the first halogen lamp heater and the second halogen lamp heater (187 and 188) have reference voltages satisfying the reference voltage of the first halogen lamp heater is smaller than the reference voltage of the second halogen lamp heater (187 and 188), the reference voltages being set in accordance with densities of contained halogen gases, wherein said reference voltage is a voltage to be applied to a halogen lamp heater so that a halogen cycle can be performed in the smoothest manner without any chemical attack, and has a value that is set for each halogen lamp heater in accordance with its density of the contained halogen gas; and wherein when a lower voltage than the reference voltage of a halogen lamp heater is applied, a halogen cycle might not be smoothly performed, and a chemical attack might occur; wherein the control unit, in use, calculates the output Duty from the temperature of the fixing member detected by the temperature detecting unit and a predetermined target temperature, calculates an effective voltage from the calculated output Duty and a rated voltage of the first halogen lamp heater, when the calculated effective voltage is higher than the reference voltage of the first halogen lamp heater, performs the half-wave control with the calculated output Duty, when the calculated effective voltage is lower than the reference voltage of the first halogen lamp heater, determines whether a temperature change detected by the temperature detecting unit shows a tendency to rise or whether the temperature change shows a tendency to drop, when the temperature change shows a tendency to rise, sets the output duty at 0, and, when the temperature change shows a tendency to drop, performs the half-wave control with the output Duty equivalent to the reference voltage of the first halogen lamp heater. a fixing member; a pressure member pressed against the fixing member; a plurality of halogen lamp heaters (186 to 188) configured to heat the fixing member; a temperature detecting unit configured to detect a temperature of the fixing member; and a control unit configured to perform half-wave control on at least one first halogen lamp heater of the halogen lamp heaters (186 to 188) by calculating an output Duty in accordance with the temperature detected by the temperature detecting unit and performing energization or de-energization for each half-wave of an AC waveform in accordance with the output Duty, and perform switch on/off control on a second halogen lamp heater (187 and 188) of the halogen lamp heaters (186 to 188), the second halogen lamp heater (187 and 188) being other than the first halogen lamp heater, characterized in that the first halogen lamp heater and the second halogen lamp heater (187 and 188) have reference voltages satisfying the reference voltage of the first halogen lamp heater is smaller than the reference voltage of the second halogen lamp heater (187 and 188), the reference voltages being set in accordance with densities of contained halogen gases, wherein said reference voltage is a voltage to be applied to a halogen lamp heater so that a halogen cycle can be performed in the smoothest manner without any chemical attack, and has a value that is set for each halogen lamp heater in accordance with its density of the contained halogen gas; and wherein when a lower voltage than the reference voltage of a halogen lamp heater is applied, a halogen cycle might not be smoothly performed, and a chemical attack might occur; wherein the control unit, in use, calculates the output Duty from the temperature of the fixing member detected by the temperature detecting unit and a predetermined target temperature, calculates an effective voltage from the calculated output Duty and a rated voltage of the first halogen lamp heater, when the calculated effective voltage is higher than the reference voltage of the first halogen lamp heater, performs the half-wave control with the calculated output Duty, when the calculated effective voltage is lower than the reference voltage of the first halogen lamp heater, determines whether a temperature change detected by the temperature detecting unit shows a tendency to rise or whether the temperature change shows a tendency to drop, when the temperature change shows a tendency to rise, sets the output duty at 0, and, when the temperature change shows a tendency to drop, performs the half-wave control with the output Duty equivalent to the reference voltage of the first halogen lamp heater.