An image forming device includes an image forming unit and a thermal fixing unit. The image forming unit uses a plurality of different color developing agents to form a color image on a recording medium. The plurality of different color developing agents are configured from binding resins with the same thermal properties. The plurality of different color developing agents include at least a high-resistance developing agent and a low-resistance developing agent. The high-resistance developing agent has a higher electrical resistance than the low-resistance developing agent and a lower charge-to-mass ratio than the low-resistance developing agent. The thermal fixing unit thermally fixes the color image onto the recording medium.