Abstract: An image-forming device includes an image-bearing member, a transfer unit, a bias applying unit, a pair of conveying rollers, a first sensor, a second sensor, a determining unit, and a controller. A developer image is formed with developer on the image-bearing member. The transfer unit transfers the developer image formed on the image bearing member onto a sheet of paper at a transfer position located between the image-bearing member and the transfer unit. The bias applying unit applies a transfer bias to the transfer unit. The pair of conveying rollers conveys the sheet of paper to the transfer position. The first sensor detects an electrical property of the pair of conveying roller. The second sensor detects an electrical property of the transfer unit. The determining unit determines an optimal bias for an ambient condition based on both of the detection results of the first sensor and the second sensor. The controller controls the bias applying unit to apply the optimal bias to the bias applying unit.

Abstract: An image forming apparatus 1 has a transfer unit, a power supply unit that takes a value of a current imparted to the transfer unit as a control target and that supplies power conforming to the value of the control target to the transfer unit; and a determination unit that acquires a detection value of a voltage detection unit and a detection value of a current detection unit and that determines, as a control target value for the power supply unit, an optimum value for the control target derived from a characteristic line showing a relationship between both detection values or a detection value of the voltage detection unit and an optimum value of the control target. The characteristic line is a line showing a relationship of a change in the absolute value of the optimum value for the control target shifting from an increasing tendency to a decreasing tendency with an increase in both detection values or the detection value of the voltage detection unit.