An image-forming device has an image-carrying unit, a transferring unit, a power-supplying unit, a current-detecting unit, a voltage-detecting unit, a memory unit, and a controller. The power-supplying unit supplies voltage and current to the transferring unit to transfer a toner onto a recording medium. The current-detecting unit detects the current flowing in the transferring unit. The voltage-detecting unit detects the voltage applied to the transferring unit. The memory unit stores a characteristic curve indicating a correlation between an optimal current and at least the detected voltage. The optimal current enables the transferring unit to transfer the toner supported on the image-carrying unit onto the recording medium. The controller controls the power-supplying unit to supply the optimal current to the transferring unit.