An image-forming device includes an image-bearing member, transfer unit, bias applying unit, a pair of conveying rollers, first sensor, second sensor, determining unit, and controller. An image is formed with developer on the image-bearing member. The transfer unit transfers the 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 conveying rollers convey the sheet of paper to the transfer position. The first sensor detects an electrical property of the conveying rollers. The second sensor detects an electrical property of the transfer unit. The determining unit determines a bias for an ambient condition based on the detection results of the first sensor and the second sensor. The controller controls the bias applying unit to apply the bias to the bias applying unit.