An image forming device performs an image forming operation in either one of a color mode or a monochrome mode. The image forming device includes a plurality of photosensitive members, a plurality of collecting members configured to collect excrescences from the corresponding photosensitive members, and a reverse-transfer member that performs a reverse-transfer operation to transfer the excrescences collected by the collecting members back onto the photosensitive members and further onto an endless bearing member. A control unit controls the reverse-transfer member to perform the reverse-transfer operation at a frequency when image forming operations are performed in succession in the monochrome mode. The control unit sets the frequency based on an image forming condition.