In an image-forming device, a controller controls the value of an effective developer bias that is established between the electric potential of a latent image formed on an image bearing body and the electric potential of a developer bearing body, to thereby develop the latent image with a non-magnetic, single-component developer agent. The controller sets a target transmission density in a region in which the transmission density of the developer agent is in a proportional relationship with respect to the effective developer bias, and controls the value of the effective developer bias based on the target transmission density and the proportional relationship between the transmission density and the effective developer bias.