An image forming apparatus having two or more different rated voltages, the image forming apparatus comprising: a voltage detecting unit, which detects a voltage level of alternating current power supplied from outside of the image forming apparatus; a control unit, which outputs a control signal according to the detected voltage level; a fusion driving circuit, which controls a number of waveforms and a phase of the AC power according to the control signal and outputs the controlled AC power as fusing power; and a fuser including: a heat generating body which receives the fusing power and generates resistance heat; and a heating member, which is heated by the resistance heat generated by the heat generating body and fuses an image formed on a printing medium; characterised in that: the heat generating body has a negative temperature coefficient characteristic; and the control unit outputs the control signal, such that the fusion driving circuit controls the number of waveforms of the AC power and outputs the controlled AC power as the fusing power during a warm-up interval until the temperature of the heating member reaches a predetermined temperature below a target temperature to perform fusing, before the fusing is performed, and controls the phase of the AC power and outputs the controlled AC power as the fusing power during the warm-up interval but after the temperature of the heating member has reached the predetermined temperature below the target temperature. a voltage detecting unit, which detects a voltage level of alternating current power supplied from outside of the image forming apparatus; a control unit, which outputs a control signal according to the detected voltage level; a fusion driving circuit, which controls a number of waveforms and a phase of the AC power according to the control signal and outputs the controlled AC power as fusing power; and a fuser including: a heat generating body which receives the fusing power and generates resistance heat; and a heating member, which is heated by the resistance heat generated by the heat generating body and fuses an image formed on a printing medium; a heat generating body which receives the fusing power and generates resistance heat; and a heating member, which is heated by the resistance heat generated by the heat generating body and fuses an image formed on a printing medium; characterised in that: the heat generating body has a negative temperature coefficient characteristic; and the control unit outputs the control signal, such that the fusion driving circuit controls the number of waveforms of the AC power and outputs the controlled AC power as the fusing power during a warm-up interval until the temperature of the heating member reaches a predetermined temperature below a target temperature to perform fusing, before the fusing is performed, and controls the phase of the AC power and outputs the controlled AC power as the fusing power during the warm-up interval but after the temperature of the heating member has reached the predetermined temperature below the target temperature. the heat generating body has a negative temperature coefficient characteristic; and the control unit outputs the control signal, such that the fusion driving circuit controls the number of waveforms of the AC power and outputs the controlled AC power as the fusing power during a warm-up interval until the temperature of the heating member reaches a predetermined temperature below a target temperature to perform fusing, before the fusing is performed, and controls the phase of the AC power and outputs the controlled AC power as the fusing power during the warm-up interval but after the temperature of the heating member has reached the predetermined temperature below the target temperature.