A fixing device includes a fuser member, a heater, a thermometer, and a power supply controller. The fuser member is subjected to heating. The heater is adjacent to the fuser member to heat the fuser member. The thermometer is adjacent to the fuser member to detect an operational temperature of the fuser member. The power supply controller controls power supply to the heater by adjusting a duty cycle. The controller includes a duty cycle calculator, a driver circuit, and a duty cycle modifier. The duty cycle calculator is operatively connected to the thermometer to calculate a primary value of the duty cycle based on the operational temperature. The driver circuit is operatively connected to the duty cycle calculator to supply power to the heater according to the duty cycle. The duty cycle modifier is connected between the duty cycle calculator and the driver circuit to modify the duty cycle.