To provide an image forming device which can set electrification conditions easily and high precisely without being affected by it even in the case that electrification characteristic changes.

This image forming device is provided with an electrifier 2 for electrifying an image carrier 1 to a required surface potential in order to form an electrostatic latent image. Currents or voltages of different values are applied to the electrifier 2 in order and the surface potential of the image carrier in each electrified state is detected by an image-carrier surface potential detection means 41. Based on each of the detected surface potentials, the value of the current or the voltage to be applied to the electrifier 2 is calculated through a linear approximation in order to obtain the required surface potential for the image carrier. The value of the current or the voltage to be applied to the electrifier 2 is determined slightly smaller than the one calculated through the linear approximation in order to obtain the required surface potential for the image carrier.